Weird Humming Noise

[51ccpe]

Im getting some weird humming coming from the back of the car. It gets louder as the car increases speed. At first i thought it might be the tires so i replaced them and it didnt go away. Now im thinking differential maybe? What do you guys think?

[TOMNGAI]

Were the wheels rotated and balanced when the new tires went on? I'd check the brakes, and hub before thinking anything else.

I'm not real familiar with diff problems, try using the search and see if anything comes up with that.

[51ccpe]

thanks. yeah i dont have too much experience with differentials so im stuck. But ill try to search around maybe someone has had a similar problem.

[ekib]

Does the noise change under any of these conditions?
- accelerate
- brake
- engine brake (in gear not touching the brake or throttle)
- cruise (constant throttle)
- turn

[elgaucho]

Im getting some weird humming coming from the back of the car. It gets louder as the car increases speed. At first i thought it might be the tires so i replaced them and it didnt go away. Now im thinking differential maybe? What do you guys think?

Must be the weather!I've got ''the noise'' too

[Meatball546]

You can try replacing the diff oil if it has a lot of miles on it; it couldn't hurt. If that doesn't do it, your other culprits are the wheel bearings, CV joints, and the driveshaft support bearing. Wheel bearings you can check by looking for play in the wheel when it is off the ground, but if all you are hearing is a humm, I don't think they have worn enough to develop play. The noise will also get louder or go away when you turn a corner.
This kind of noise is not typical of CV joints, but I wouldn't rule it out just yet.

[51ccpe]

I finally got around to check all the stuff you guys mentioned. If the car is in park, it is very quiet. Doesnt hum under braking either. The noise starts up at around 15 mph and the faster i go the louder it gets (a very obnoxious loud).
On turns, the humming stays consistant no change really.
When the humming first started it would start around 40 mph get louder till about 82 mph and @ 90 mph+ it would go away. Now its constant all the way until i let off the gas. Thanks for your help guys.

[CoolJoe]

I place my bet on the rear wheel bearing. I had the exact humming noise.

[BMW525i]

I place my bet on the rear wheel bearing. I had the exact humming noise.

+1 sure sounds like a wheel bearing

[sar5036]

+2

[51ccpe]

Thanks fellas. Im gonna get the bearings checked out this weekend. Cant have the bimmer running like crap.
Btw, does anyone have a diy for the bearings?

[Meatball546]

It's a pretty difficult job to do, requiring some specialized tools. If you don't have the right tools, it will be much more difficult. It's not impossible for the dedicated DIYer though! http://forums.bimmerforums.com/forum/showthread.php?t=642032 Firstly, however, I'd recommend verifying it's actually the wheel bearings making the noise. If you jack up the rear and can't find any play in the wheels, keep driving on it. If they are about to give up, the noise will eventually turn into a grinding that will get louder as the car's weight is shifted to that side and the bearing is loaded, and it will go away or get quiet in the other direction. Then you can be sure it's the wheel bearings that need to be replaced. It's a lot of work to do if the bearings aren't really bad.

[51ccpe]

Good point, thanks. I'll check it out this weekend.

[benemorius]

It might be worth your while to do a bit more investigation before throwing money at it. It could very well be a wheel bearing, but the fact that the noise is constant while turning does cast a bit of doubt on that. If you put the car up on jack stands and have a buddy get the wheels up to speed, you can listen more closely and find out pretty easily whether the noise is coming from the diff, one wheel bearing, or the other. Or if you know a friendly mechanic, he could likely tell you what it is just by going for a quick ride. Bearings and diff gears and diff bearings make different noises and an experienced ear can tell them apart rather efficiently.

[51ccpe]

True. Sounds like i got some work to do this weekend....

[hellrot325]

Doubt it's a wheel bearing. COuple other...and cheaper thoughts...guibo or center bearing, or it could be just the insulation laying on the driveshaft.

[onysik22]

I think I have the same noise you mentioned, though I've never been able to determine where it was coming from myself. I would describe it more of a like a whir, rather than hum. Happens only at speeds ~15-30, at rpms from around 2-3k but no higher. Doesn't seem to be something that is going to crap out soon but I'd like to know what it is and what I should do about it so if someone could report back or give a more definitive answer here I would appreciate it.

[somesnapper]

Drive shaft!!

[Tamikinns]

Would my humming noise be a drive shaft problem? I hear the noise only when I'm over 50 mph on the highway..but notice it dissapear when I turn my wheel to the right on a curve. Once Off a curve and the road is straight it comes back..I thought maybe my tire was rubbing or something but don't see any damage to my tire or the inside wall of my back fender.

[hellrot325]

Would my humming noise be a drive shaft problem? I hear the noise only when I'm over 50 mph on the highway..but notice it dissapear when I turn my wheel to the right on a curve. Once Off a curve and the road is straight it comes back..I thought maybe my tire was rubbing or something but don't see any damage to my tire or the inside wall of my back fender. That is likely a wheeel bearing.

[Tamikinns]

Ok, since I know NOTHING about cars..how much would you say I am looking at the damage to my bank? lol. IS that bad?

[hellrot325]

Ok, since I know NOTHING about cars..how much would you say I am looking at the damage to my bank? lol. IS that bad?$300-400? If it is a bad wheel bearing...do both sides...if one went, other is generally about to. Rears are more expensive...harder job.

[Tamikinns]

Aww..man! Thanks!!! :)

[ngcreese]

I have a 1991 325i 2 door 5spd stick.
It had roughly 176K on it when I bought it. I've been renovating it over the course of time. Done so far:

Bilstein Shocks & Eibach Springs.
Eibach front and rear anti-sway bars.
Bav Auto rigid strut tower brace.
New rotors & pads front and rear.
Alpine head unit, 4 ch amp, mono sub amp & 12inch JL Audio sealed sub

---(here's where it pertains to this thread)---
New front hubs w/ bearings.
New rear wheel bearings. (I used a slide hammer rented from Advanced Auto Parts to pop them out, and install the new ones).
Dropped the regular diff out, put a posi-traction diff in.
--------------------------------------------

Tranny service being done today:
New throw out bearing (failed),
New Clutch, Rear Tranny seal & Guibo ( mostly preventative, might as well, with the tranny out of the car ).

My question is...
Before I did anything to the car, it had that constant metallic (almost like fm-radio static) sound in the back ground. The one that's noticeable when you drive faster than 30 mph, gets louder as the car goes faster, doesn’t change or go away if you turn left or right (except of course, if you’ve slowed down to make the turn…), and seems like its coming from something in the rear of the car.

I've changed the diff oil in both the regular & posi-diffs, no difference.
Wheel beerings are new, no difference.

Some folks have said, it could be the tranny or the center drive shaft bearing.
Any thoughts on this?

The guys will be checking for this while they're servicing the car.

Could it be the half shafts (CVs) running from the diff, out to the wheels?

While on the topic of the drive line...
I hear a "clank" when I put the car in reverse and let the clutch out, I'll get another "clank" when I put it back into 1st gear and let the clutch out.

The shop guys feel the Guibo may be contributing to this... but my driveshaft definetly has some play in it, and the clank could probably be coming from there.
Bav Auto has drive shafts for about $650.00 and shop wants about $300 to install it. Hmm...

Anyone made progress on locating that constant metallic-static rear end sound?

If so, please post… I know I will be following up if the shop solves this problem.

[Bris328is]

I would say its your trunk hooker, she is hungry for a whopper--if you have not gotten the trunk hooker upgrade, then its definately a wheel bearing, thats the noise mine made.:D

[ngcreese]

Yeah, those trunk hookers can be a PITA...

Got the car back... with the new throw out bearing ( and new clutch, guibo & rear seals) car runs quieter... there still is some back ground road noise / static...

Mechanic at the shop points to changing the drive shaft and it's corresponding center bearing as my next upgrade.

Though I haven't found the ultimate source of the back ground metallic-static... Fixing the throw out bearing helped. Also, looks like the drive shaft is the next most likely candidate to help quiet things down.

After doing some internet wide searching...

I think a better discription of the noise I'm hearing is a constant grinding noise... which gets mistaken as something dragging against a break rotor.. or... a bad wheel beering.

However, what it could end up being is worn out CV joints. One mode of failure for a worn CV joint is for it to put out a constant grinding sound... especially if it had a torn boot that was not caught early... and the car DID have at least one torn boot when I bought it... hmmm...

The mechanic who fixed that boot, I never trusted... especially due to him being pissed off cause I took a weekend to replaced the entire suspension, front control arms & bushings, rotors & pads, and all the wheel bearings in the car myself, prior to getting it inspected at his shop... he missed out on $6K worth of work.

He DID charge me $700 to replace the CV BOOT and a power steering hose!! (If i only had more time, I would have done those myself too espcially since I already had the cv joints out to replace the rear wheel bearings.... GRRRR!!!) I could buy a new CV joint for a little more than that.
Needless to say, that was the first and last time I took my car to that shop.

Weekend experiment...
1. Jack up the rear end of car,
2. Start car & put in 1st gear,
3. Get under car and listen to cv joints.
4. Put in neutral.
5. Shut down car.
6. Disconnect CV Joints from diff (there's only 6 bolts per side on an '91 E30).
8. Repeat steps 2 to 5...
9. Reconnect (old) CV joints
10. If I don't hear grinding noise... I'll buy a new pair of CV joints (331.00 each).
11. If I still hear grinding noise... it's my diff or the drive shaft.

Will update again once this is done.

[ngcreese]

Well I've executed on the experiment.
Jacked up the rear of my car and disconnected the half shafts from the differential. In this way, the only things spinning on the driveline are the transmission, main driveshaft and the differential.

With the car's rear jacked up, I had a friend start her up, put her in 5th gear and give her enough gas to put the speedometer up around 60 mph. That's about 2500 rpms. I got under her and listened.

The constant metallic grinding noise and only varies volume and/or pitch with the car’s speed, is the main driveshaft's center support bearing.

With the driveline spinning at what would be 60mph under normal circumstances, I put my ear up to the differential, just to be sure. The diff was quiet. When the driveline wasn't spinning fast (engine at idle, with tranny in 5th gear), it was easy to mistake the center bearing noise for diff noise. I suspect this was, because the noise coming from the bad bearing, is coming from inside the driveshaft's tunnel, which opens up right at the differential.

With the driveline spinning at high speed the howling driveshaft center bearing can't be mistaken. The center bearing is LOUD, when you're under there. This makes me appreciate just how MUCH sound deadening there is, on the floor board of my car. By comparison, from inside the car, at 60mph, the same noise source is much quieter.

I did take the opportunity to install new half shafts, since they're really old and only one additional nut and a little patience with a hammer, was all that was needed to make the swap. Not to mention, it's way cheaper than having a shop do it.

So... next upgrade... new main driveshaft, w/ new center bearing.

I'll have a shop do this one, since I’m thinking a driveshaft swap is way more labor intensive.

I hope someone finds this info helpful.