Alternator whine... but not? Insight needed

[Aubergine]

Hi All,

I've been trying to diagnose a very strange noise on a `97 528i and am looking for some insight. The noise sounds very similar to the alternator whine you would hear through speakers with a poorly wired up radio. But... it is not coming through the speakers. The whine happens only while the vehicle is in motion and gets progressively louder the faster you go. If the vehicle is stationary and rev it up, there is no noise.

In and effort to isolate the noise, I removed the driver door panel and went for a drive with all the driver door speakers disconnected, the noise persisted. If I had to pinpoint the location, I would say somewhere in the dashboard near the left front dashboard defogger vent/behind the gauge cluster. The cluster works normally and the car is not presenting with a CEL or any codes in the modules (used my handy Foxwell scanner with BMW software).

My next step will be to remove the cluster and poke around. Any ideas?

[seagreen323i]

Possibly distorted wind noise? Speaker interference will typically be rpm related not speed related. Could be speed related if it was say an abs sensor.

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[ross1]

I cannot think of a single thing inside the car that would correlate to road speed with the possible exception of the speedometer. I've heard cable driven speedos make horrible noises but never a peep from an electrically driven one like ours. Perhaps disable the speedo by finding the appropriate fuse to rule it out.

[CPHES]

Do any work on that side. Look for missing body plugs. Some have said missing jack points cause a whistling noise. There is one at each corner - look at driver side. If missing, put tape on that hole to experiment

[Aubergine]

Possibly distorted wind noise? Speaker interference will typically be rpm related not speed related. Could be speed related if it was say an abs sensor.

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This was what I was thinking. The quality of the noise is like and alternator whine but it obviously isn't that because of the speed correlation.

I cannot think of a single thing inside the car that would correlate to road speed with the possible exception of the speedometer. I've heard cable driven speedos make horrible noises but never a peep from an electrically driven one like ours. Perhaps disable the speedo by finding the appropriate fuse to rule it out.

Yes, I was thinking I'd pop out the cluster and poke around. I can drive without it plugged in and see what happens.

Do any work on that side. Look for missing body plugs. Some have said missing jack points cause a whistling noise. There is one at each corner - look at driver side. If missing, put tape on that hole to experiment

So the jack pads are all there but the engine under panel was damaged so I removed it. I've driven without them on other E39s and never had the noise.

[Ed CT]

So, just to be clear, the noise is speed related - it gets louder as speed is increased.
Does the pitch of the noise change (go up) as speed increases? Or is it a constant pitch only louder? If the pitch increases with speed, then it's wheel speed/drive shaft related. If it does not change pitch, then possibly wind-related, maybe thru the ventilation system.

[SW530]

Nothing in the car will make a noise that changes with speed. I'd take a close look at your serpentine belt and even remove it and see if the noise persists. The car will be okay for a short test without the belt. If the noise stops you know it's the belt of one of the accessories, Probably the alternator.

[BimmrMeUpSnotty]

OP SAID SPEED RELATED. So it’s got nothing to do with things that are constantly spinning once started. So, that narrows it down to things that spin once moving. So, wheel bearings, brakes, drive shaft, again, bearings! The center bearing support is my bet, rear diff, if it is low on oil, it will howl, rear wheel bearings. The noises he describes, are the kinds of elusive noises that may sound like it’s coming from a certain spot in front, but in reality, could be coming from the rear.

[SW530]

Don't know about your E39 but the engine in mine definitely spins faster as you go faster.. figure that's kinda how it works..

[CPHES]

Not likely for him, but I had a bad idler pulley on the Yukon. As the rpm’s changed and dropped as it shifts up, the noise changed. Less noise at 60, than 30. Yes check rotating assemblies

[SW530]

What I'm really thinking is the alternator rectifier bridge has a partial failure that's letting feedback through the ground which means it could be coming out of any/all speakers and microphones like the one possibly installed for the hands free feature. Its a long shot but I've seen it before. My very first job out of Voc Auto was to be the squeak and rattle guy. Absolute lowest position on the totem pole. It didn't take long to realize sound in a vehicles interior is rarely directional and to learn to trouble shoot by eliminating different systems. There is a professional version of this that uses BT instead of wires and is a very useful tool to find errant sounds. I wish someone made one of these kits with video as well as you can often see noises via movement or vibration.

[seagreen323i]

I say it's not electrical feedback related and if it is its an abs tone ring signal.

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[Aubergine]

Hi all,

Catching up on the replies! Lots of good ideas and I've added more info below.

So, just to be clear, the noise is speed related - it gets louder as speed is increased.
Does the pitch of the noise change (go up) as speed increases? Or is it a constant pitch only louder? If the pitch increases with speed, then it's wheel speed/drive shaft related. If it does not change pitch, then possibly wind-related, maybe thru the ventilation system.

Hi Ed, To add more info, the noise is directly related to speed and not RPM. If I am doing 40 MPH in 2nd gear the noise will be the same as if I am doing 40 MPH in 5th gear.

The driver side cabin air filter duct had come undone and I put it back properly but the noise was the same before and after. I am thinking wind noise though. The under panel was removed because the attachment points on the brake air ducts broke (long story) so the under panel is currently out of the car.

Nothing in the car will make a noise that changes with speed. I'd take a close look at your serpentine belt and even remove it and see if the noise persists. The car will be okay for a short test without the belt. If the noise stops you know it's the belt of one of the accessories, Probably the alternator.

I can pop off the belt and check. The alternator was replaced with a Bosch reman in 2020 and the idler pulley as well as the tensioner were replaced about 6 months ago. That said, it is worth confirming.

OP SAID SPEED RELATED. So it’s got nothing to do with things that are constantly spinning once started. So, that narrows it down to things that spin once moving. So, wheel bearings, brakes, drive shaft, again, bearings! The center bearing support is my bet, rear diff, if it is low on oil, it will howl, rear wheel bearings. The noises he describes, are the kinds of elusive noises that may sound like it’s coming from a certain spot in front, but in reality, could be coming from the rear.

Agreed. I think next time I investigate I will put someone in the back seat and roll down the windows. I have had diff noise on two E39s before and this doesn't sound like that, but some of the other items are possible.

Not likely for him, but I had a bad idler pulley on the Yukon. As the rpm’s changed and dropped as it shifts up, the noise changed. Less noise at 60, than 30. Yes check rotating assemblies

Will do!

What I'm really thinking is the alternator rectifier bridge has a partial failure that's letting feedback through the ground which means it could be coming out of any/all speakers and microphones like the one possibly installed for the hands free feature. Its a long shot but I've seen it before. My very first job out of Voc Auto was to be the squeak and rattle guy. Absolute lowest position on the totem pole. It didn't take long to realize sound in a vehicles interior is rarely directional and to learn to trouble shoot by eliminating different systems. There is a professional version of this that uses BT instead of wires and is a very useful tool to find errant sounds. I wish someone made one of these kits with video as well as you can often see noises via movement or vibration.

I replaced the alternator with a Bosch reman in 2020. Do you have info on how I would check the rectifier specifically?

I say it's not electrical feedback related and if it is its an abs tone ring signal.

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Any thoughts on how to best read this? I have the suite of BMW programs but I also feel like this is a job for an oscilloscope.

[SW530]

Not without removing the alternator but if it's putting out 14.4vdc its probably fine. If you don't have a meter you can actually (and carefully) just remove the battery cables while running. It should continue to run. But since you clarified it's speed and not rpm related I no longer think this is it. More likely wheel bearings (hubs) or tire noise. It doesn't take much of a defect with a tire to cause different noises. Possible its the drive shaft and center bearing but you can easily check that by getting up to speed where you can hear it and then put the clutch in (or shift to neutral). You didn't say auto or manual? The only constants with speed are your spun weight and rear dif. That includes all wheel bearings, tires, rotors, etc.. Process of elimination..

[Aubergine]

Not without removing the alternator but if it's putting out 14.4vdc its probably fine. If you don't have a meter you can actually (and carefully) just remove the battery cables while running. It should continue to run. But since you clarified it's speed and not rpm related I no longer think this is it. More likely wheel bearings (hubs) or tire noise. It doesn't take much of a defect with a tire to cause different noises. Possible its the drive shaft and center bearing but you can easily check that by getting up to speed where you can hear it and then put the clutch in (or shift to neutral). You didn't say auto or manual? The only constants with speed are your spun weight and rear dif. That includes all wheel bearings, tires, rotors, etc.. Process of elimination..

Thanks! I was starting to go down that path of thought. The car is manual so I'll put in the clutch. I can break out the multimeter but I suspect the alternator is giving the proper output.

The CSB and flex disc should be okay since I looked at their condition not too long ago, but I will check again and put in the clutch while at speed.

[SW530]

Do it logically and don't skip any steps. An example is I used to visually check fuses and figured what were the odds of a fuse being bad if I could clearly see it's intact. Turns out better than you might think. So, now I meter each fuse for the suspected circuit. It really sucks to do hours or even days of work and many dollars worth of parts because you glossed over something you should have checked.

[ross1]

Anything on the engine will make the noise if moving or not with the possible exception of an intake leak(I'm thinking CCV)that will see different vacuum levels when under load.
Have you determined if it coming from in the car, under the hood or elsewhere?
All road speed related suspects will be "south" of the transmission output flange as these items are the only ones that rotate in direct proportion to wheel(road)speed. So, drive shaft, it's related bearings, differential and axle shafts, wheel hubs and brakes.
One thought is a bent or loose brake backing plate scraping on a rotor. I, myself, wouldn't call that noise a whine but perhaps others would.

[Aubergine]

Do it logically and don't skip any steps. An example is I used to visually check fuses and figured what were the odds of a fuse being bad if I could clearly see it's intact. Turns out better than you might think. So, now I meter each fuse for the suspected circuit. It really sucks to do hours or even days of work and many dollars worth of parts because you glossed over something you should have checked.

I do the work myself and it is just the time so I'll trace it down. I may pull the cluster just to take a peek and satisfy my curiosity.

Anything on the engine will make the noise if moving or not with the possible exception of an intake leak(I'm thinking CCV)that will see different vacuum levels when under load.
Have you determined if it coming from in the car, under the hood or elsewhere?
All road speed related suspects will be "south" of the transmission output flange as these items are the only ones that rotate in direct proportion to wheel(road)speed. So, drive shaft, it's related bearings, differential and axle shafts, wheel hubs and brakes.
One thought is a bent or loose brake backing plate scraping on a rotor. I, myself, wouldn't call that noise a whine but perhaps others would.

When I revved up the car while standing still I could not get it to occur so I am doubtful that it is coming from the engine itself. The Engine was replaced about 8 months ago by a BMW Master Tech friend and the CCV valve/pipes were replaced at that time.

I am still of the mind that it may be a wind noise so I have a few theories to check.

[Ed CT]

Aubergine - Maybe have someone stand on the roadside as you drive by so they can listen for the noise and narrow down which part of the car it's coming from.

[Aubergine]

Aubergine - Maybe have someone stand on the roadside as you drive by so they can listen for the noise and narrow down which part of the car it's coming from.

I was thinking I may do that! I'll let everyone know my findings.

[BimmrMeUpSnotty]

Do you have roof racks on the car? If so, are the cross beams airfoiled? If so, what direction do you have the airfoil aiming? If the airfoil is facing the wrong direction, you will definitely get a loud whistle that gets louder, the faster you go.

[Aubergine]

Do you have roof racks on the car? If so, are the cross beams airfoiled? If so, what direction do you have the airfoil aiming? If the airfoil is facing the wrong direction, you will definitely get a loud whistle that gets louder, the faster you go.

Good thought! I do have roof bars for the car... but they are not currently mounted. The hinged doors for the mounting points are all in the closed position as well.

[E0339]

As the car warms up, does the noise subside?

Does the noise change as the car goes up in gears?

I had a whining sound when my car was cold. When it warmed up it went away. I thought it was coming from the accessories of the engine. Fan. Some pulley. But nope. Tranny oil was low. Real low. Make sure you check oil level at proper temperatures, or it will read much higher than it actually is.

[Aubergine]

As the car warms up, does the noise subside?

Does the noise change as the car goes up in gears?

I had a whining sound when my car was cold. When it warmed up it went away. I thought it was coming from the accessories of the engine. Fan. Some pulley. But nope. Tranny oil was low. Real low. Make sure you check oil level at proper temperatures, or it will read much higher than it actually is.

Thanks for the suggestion! The noise does not change as the car warms up. I am going to take some time to run through these suggestions over the long weekend and I will report back.

[Aubergine]

Time for and update!
I went through the list of items this could be and determined that it is caused by wind noise. The noise continued to be speed dependent and was slightly louder when my friend and I removed the driver side cabin air filter housing, duct, and left side windshield drain (smaller triangular piece that snaps in to the outer edges of the lower windshield cowl/wiper mechanism cover. The part that I failed to mention because I thought it was unrelated was that the car was missing the center firewall cover in the engine bay (that the engine wiring harness routes through) as it had broken when being removed to replace the engine. It was not troublesome until there was a unfortunate incident with an ice bank that took out the lower section of the front bumper and the engine under tray. With the under tray and firewall cover missing air is coming through the engine bay and right though the wiper mechanism and windshield cowl areas. There are plenty of sharp edges that allow it to produce a whistling noise that is speed dependent.

I will replace the under tray and firewall cover in a few weeks and provide another update!