1990 735i "drone" sound with minor vibration

[E24 nokaoi]

Good morning everyone:

I have owned my prize 90 BMW 735 for a long time and it is the best one I have ever owned, and I have owned many Bimmers over the years. This fine machine is having it's 26th Birthday this month and has finally arrived at the 100K mile mark! And it still looks close to new. I have noticed in the last few years it is getting a louder "droning" sound, heard mostly from inside the car. Outside it has the normal BMW exhaust note. I also feel a slight bit of vibration with it. Last year I replaced the rear muffler (or resonator, whichever is the appropriate term) with the OEM German one, which I was lucky enough to find at a supplier in California. No change in the drone sound whatsoever.

The forward muffler (sorry, "silencer") is the original and has no leaks and looks perfect, from the outside. I would replace it if I could find a source for the original, other than ordering it from the Dealer. One thought I had is the possibility that the motor mounts (which I replaced 8 years ago) might have gone bad. Another thought is that the tranny mounts might be bad, as I have never changed them. The sound/feel makes me think the engine is no longer isolated from the body to the extent it used to be.

The more load it has, the louder the sound gets. On coasting, the car is dead quiet. Of course, it may be the center silencer, but it is only really noticeable inside the car, a resonate kind of sound. I don't want to try one of those crappy aftermarket mufflers to find out, since they don't last long. I mentioned this to the shop that replaced the rear muffler and he said the headpipes might have become thin over the years, even though the mileage is low. That is a theory I had not heard before, but he seemed to feel that could be the problem.

Now, my question is this: Is there anyone on this wonderful forum who knows, or feel they might have reason to know, what is going on here? I plan to keep this wonderful E32 till it croaks (or I do) so anyone who can help me figure this out has my undying respect and admiration!!

Thank you.

Garry in Portlandmy garage queens0003.jpg

[shogun]

Difficult to find out without hearing it. I could imagine that a small part is slightly lose like a heat shield. For example I had this or similar sound some years back till I found that the bracket of the ATF lines from transmission to radiator was broken and under certain conditions the metal lines touched another part.
Or the center bearing of the driveshaft is at it's end and needs to be replaced.
Maybe some other members can tell you more possibilties from their own experience.

[Oleh007]

I have the same drone/vibration and it is very noticeable in the 35-45mph range. Same symptoms as yours. When weekend comes I'll try to look and see under the car if something may be loose like shogun suggested. Also if it is motor mounts, shouldn't the motor move when giving it gas? Or is that only very noticeable on the m60 like I read from other threads? And just a thought but why would the transmission mounts cause vibration? I thought that since it is connected to engine, it is always moving with the engine shaft when in drive, that means even if if you let off the gas and coast, the drone/sound should still be there? I don't know.

[shogun]

engine mounts and trans mounts get smaller or even completely rip of the metal foot. I have seen trans mounts completely separated. So a defective engine, trans, center bearing could cause a not straight position of the complete driveline, this can cause for example vibrations. Here is a pic of old and new M30 engine mount, see the height difference http://produkte.bilder-speicher.de/1...t-und-neu.html

Of course, with such defective mounts also the engine moves more in engine bay clockwise and anticlockwise depending on your throttle.
Also check for broken muffler rubber hangers, plus the heat shield above the exhaust could be lose.

[E24 nokaoi]

Hi Shogun,

Thanks for your thoughts. I have eliminated any loose heat shields, bad exhaust hangers, etc. This is not a light, metallic sound, it is a heavy, deep "drone" sound and is only present under load, which made me wonder if I could have engine or tranny mounts "bottoming out". What I am hearing seems to come from the engine compartment, or near to it. I checked the firewall to be sure there were no unplugged holes, etc. I can't believe this noise is coming from the forward silencer, even though it is the original (only 100,000 miles on the car), and the rear one is new OEM.

Is there a way to measure the height of the motor mounts to determine if they have collapsed (I don't know how they should be, with the engine resting on them)? I have an old school factory trained BMW Master tech who lives 300 miles South of me and I may take it down to him for tranny service and have him take a look at it when it is up in the air. I can't safely get it high enough to crawl under it.

This is the finest E32 I have ever owned but the deep droning sound bothers me on a road trip, but it is too nice a car to sell (and I am attached to it.

Thanks again for your help. I know you know these cars well, and I thought I did, but this one has me stumped.

[shogun]

It is probably difficult to measure as you have to take the engine mounts out. In case you never have changed the engine mounts before, just change to know. They are wear parts. Talk to your trained BMW Master Tech before you buy them, maybe he can recommend some items for your purchase list before you visit him.
Might be even a small thing which causes the noise. I once had that and it was just a broken plastic clip for the transmission pipe from trans to radiator, the clip broke = pipes could vibrate under heavy load and touched some other parts in the engine bay. Just as one example.

[E24 nokaoi]

It is probably difficult to measure as you have to take the engine mounts out. In case you never have changed the engine mounts before, just change to know. They are wear parts. Talk to your trained BMW Master Tech before you buy them, maybe he can recommend some items for your purchase list before you visit him. Might be even a small thing which causes the noise. I once had that and it was just a broken plastic clip for the transmission pipe from trans to radiator, the clip broke = pipes could vibrate under heavy load and touched some other parts in the engine bay. Just as one example.

Who do you think makes the best engine mounts for my 1990 735i (or closest to what the original factory ones were. It is very confusing when shopping on line to determine if O.E or O.E.M. in their ad means it is the same as the car originally came with, made by the same supplier BMW used on these cars. Can you shed any light on that? I try to avoid aftermarket parts in certain applications, and engine mounts is one example. When running the engine just above idle I feel a vibration on the steering wheel.Thanks for you thoughts.

[shogun]

Probably no one has tested them all on his car, so it is difficult to say which one is best. FCP has Corteco, Febi, Rein https://www.fcpeuro.com/BMW-parts/73...86&m=20&page=1
As per FCP definitions:
OEM parts are produced by a manufacturer that meets the high quality standards and produces at least one part for a vehicle manufacturer.
Aftermarket parts are produced by a manufacturer that was has not been contracted to produce parts for a car’s assembly line

I have used all these 3 brands before and had no problem. Corteco is part of the Freudenberg Group, Rein is part of CRP Automotive.

[TonyPa]

I bought Rein mounts from FCP just not long ago. Seem like very nice. No complaints from me.

[SergeBMW]

If it happens under acceleration it sounds like exhaust. Could also be the torque converter locking up too early, it causes a deep droning noise at around the same speed. Most likely due to bad fluid transfer through the TC and this is only noticed during acceleration at those speeds. Also check the exhaust connections, could have a leak on one of the joints.

[Mareczek]

Hi E24 nakoi,
I think i have the same issue as You have. Have You resolved the problem? I checked all posibilities and getting me crazy At 2000 rpm there is some kind of resonance.