Whine from the rear

[Demelich]

I recently purchased a 91 318is.  I have found other threads with similar problems, but I would like to get some other opinions too.

The problem is that over about 25MPH, I hear a whine coming from the back of the car.  It is kind of like the whine from a DC fan spinning at high speeds.  The whine increases in pitch, but not so much in volume as speed increases.  The whine seems to be directly related to wheel speed.  If I push in the clutch and shift to neutral while going 65+, the whine stays the same and slightly decreases as speed bleeds off.  Braking makes the whine reduce in pitch and volume much quicker than if I just coast.

The previous owner said that the problem started after he installed a new fuel pump, and the sound itself really does seem like it could be the fuel pump.

The problem is that the symptoms do not seem to be consistant with a fuel pump (in my opinion  anyway, Ive never had a fuel pump issue before) because even if I have the engine idling in neutral at highway speeds, the whine remains the same.   Could it be that it just takes a while for the fuel pump to reduce its speed?

I do not believe that the problem is wheel bearings either because even hard turning does not effect the whine in anyway.

I think that it may be the guibo, or something driveshaft related.  Otherwise it may be the differential itself although the diff seems to be working fine.

So, could I just be wrong and I actually have some sort of leak in the fuel system or a bad fuel pump?  Or does this sound like a driveshaft/differential issue.  

One last note, the whine remains steady no matter what gear I am in.

[hoevesruperd]

i had a similar problem, thought it was the wheel bearing but then i realized that my front wheel was really hot. my caliper was clogged and the pads where lightly grinding on the disk. before you start to worry too much, check if your wheels are excessively warm after driving.

i'm in no way assuming this is your problem, it could be anything wheel related. if you say that the sound keeps on going and keeps its pitch when the car is in neutral while rolling i doubt it would come from the differential or the shaft as these go with the rpm

[Febi Guibo]

my money is on the diff (output bearing going bad)

[RED IS 91]

my money is on the diff (output bearing going bad)

+1 ^

[Demelich]

I have been thinking diff also Febi Guibo.  I assume the diff needs to be dropped for pretty much any maintenance.  Is the output bearing something that can be done, or is it cheaper and easier to buy a new rear end?  It seems to me that in most cases it is.

On the other hand, the previous owner also put on new pads and rotors.  The brakes squeek a bit, so its possible he botched that job.  I will check that out, but the noise is definately coming from the back, not the front.

[hoevesruperd]

i'm in no way assuming this is your problem, it could be anything wheel related. if you say that the sound keeps on going and keeps its pitch when the car is in neutral while rolling i doubt it would come from the differential or the shaft as these go with the rpm

just realized i'm totally wrong here

[nicknikolovski]

I got a 93 318is coupe and I just started noticing that same noise but only from the right hand rear side. Let me know what your outcome may be?

[DZick]

If it is a whine all the time (even when not moving) probably fuel pump going bad. they usually get loud when they are about to go out.
 
Other wise if it does it only when you move id bet on the diff.

[hoevesruperd]

If it is a whine all the time (even when not moving) probably fuel pump going bad. they usually get loud when they are about to go out.
 
Other wise if it does it only when you move id bet on the diff.

remove your back seat you ll be able to tell in 2 seconds if its your fuel pump. but fuel pumps are more of a buzz sound than a whine.

[Demelich]

Its a fresh fuel pump on mine, of course its possible the previous owner screwed up when installing it somehow.

I am about 90% sure its the diff, but Ive had both the previous owner and my dad try to tell me its the fuel pump.

I am just about to call a shop to check out the diff, so I will see what they have to say.

EDIT: Just a quick update, I have not had a chance to contact the shop yet.  I only work till noon today though, so I am going to head over today.

[Asserti]

Got the same problem. But I located the sound to come from right under my passenger ash tray. I used pieces for the cardan from two different cars. I think the connection between the two pieces makes the sound.

[cheesebox]

It's also possible part of the hand brake assembly has come adrift in one of the rear drums.

To verify, drive until you hear the sound, then gradually pull on the hand brake. If the sound disappears then that is your problem.

[sheepdog]

Does it change based on acceleration vs deceleration? That would be gear whine.

If it changes only based on speed it is a bearing.
Could be diff or wheel, have a buddy sit in back he may be able to tell which.

[Demelich]

So an update (finally).

I have done a bit with my car since then.  A bunch of new suspension parts, control arms, springs, shocks etc.  Anyway, more importantly, I have replaced both rear wheel bearings, and put in a 4.10 LSD from diffsonline.com.  One of my rear calipers locked up as well, so my rear calipers, pads and rotors were replaced.

The whine is still there.  It really sounds like it is coming from the back, but I wonder if my driveshaft is making the noise.  My ebrake seems to work properly, so I do not know if that could be it.

Before replacing the diff, I lifted the rear wheels up and spun them up to about 60MPH.  My mechanic friend said that the noise was coming from the diff.  Since I have a new (used) diff and the noise is still there, I am looking towards something with the driveshaft.

I think my next step is to check out the Guibo/flex disk and center bearing.

I wonder, could it simply be my tires making noise?  Sounds more mechanical/metal than a rubber on pavement sound though.

[r0ckrat]

Sounds like a possible center support bearing on the driveline - back under the rear seat, but not as far back as the diff.