Rear Oscillating Noise

[4tDX]

Hello,

I am having trouble diagnosing a problem I've been having on a 1987 325i. Nothing I've read or listened to matches the noise I'm hearing, which why I am now starting this thread...

The noise is hard to explain... The noise is oscillating relative to the speed of the car and by that I mean the volume of the noise quickly rises and falls. It is more of a whirring noise, like something is off center. Slight vibration some times accompanies the noise. Sounds like it is coming from the rear center or rear right (passenger side) of the car.

It initially started ~2months ago and would only occur under very hard left-hand cornering and would occur more frequently with throttle being applied. Recently is has been happening more frequently on left-hand turns and it now some times occurs even while going straight at high speeds (+60mph).

I had first thought it was the right axle, so I replaced that with a new one (GKN) and threw on a new rear bearing too. Also, over the past year I've basically been refurbishing the car: front and rear suspension bushing and ball joint all replaced, front wheel bearings, drive-shaft guibo, new fluid in LSD, axle-diff bolts, diff mount, transmission mounts, engine mounts, etc.

I still need to take a closer look at the drive-shaft bearing and u-joint, but it seems odd that the noise would only start occurring on left-hand turns and not right-handers too. I'm also worried it may be related to one of the differential output shafts.

Does this sound like something anyone else has dealt with?

Thanks!

[jlevie]

Generally speaking a noise that occurs when turning in one direction, but not the other with be with the "loaded" side of the car. In other works if the noise occurs when turning right, the problem will be on the left side and vice versa.

Since the noise occurs on left hand turns and since you have already replaced the right hand bearing and half shaft, my next suspicion would be the differential.

[4tDX]

Since the noise occurs on left hand turns and since you have already replaced the right hand bearing and half shaft, my next suspicion would be the differential.

Yeah, that was my fear. Guess I'll source an open diff and see if that eliminates the noise. Thanks.

[audiquattrot]

I had a similar issue, and it ended up being the fan clutch/blades. Some of the blades on the fan broke off, and made it unbalanced. I experienced the 'whirring' sound too.

[doorman]

I'm having a similar issue.
Did both rear bearings, one was definitely bad, replaced both axles at that time and diff fluid.

Sound isn't horribly loud, radio/windows open conceal most of it.

I have a newer diff coming soon and will report back afterward.

Car does have approx 350,000+ miles on her.

[4tDX]

I threw an open diff in there this weekend and still hearing the noise.

I'm now leaning towards it being drive shaft related. I hope to have the time later this week to investigate it before I buy anything. I know the guibo is new (and installed correctly), so I am thinking something to do with the CSB.

Some new questions... When I removed the diff the rear drive shaft easily came out. Should it be very easy to pull out? I was under the impression that the nut should keep it fairly secure.

Also, will the small (late model year) CSB bolt onto an early model car or do I need to get the early model CSB? I believe I do have the small output flange on the transmission ('87 manual).

[jlevie]

Take a good look at the rubber mount of the CSB. If it is torn or cracked the cause will almost certainly be binding or loose u-joints in the drive shaft. A new CSB might give temporary relief, but the long term fix will be a rebuilt drive shaft, which typically comes with a new CSB installed. I like those from Driveline service or the ones that Bimmerworld offers.

The collar nut is really just a grease seal. If in good condition the halves should come apart easily. You have to use the CSB meant for the car. A late model CSB won't work on an early car or vice versa.

[4tDX]

Just thought I'd update this for anyone experiencing similar issues.

Finally got around to taking the drive shaft out and CSB looks shot. No cracks or tears, other than surface cracks that just seem to be from old age. However, rubber is completely shot and is providing no real support. It has drooped/deformed downward so that the bearing / shaft is no longer centered in the support. The bearing also sounds and feels completely shot.

Just to summarize, the noise was an oscillating whirring noise that was occasionally accompanied by vibration. The noise would occur during left-hand turns while throttle was applied. The noise would also occur when the rear end experienced quick deloads such as quick abrupt crests.