Our new-to-us '99 328i auto, 100k, has a clunk in the rear-end/diff area. Typically at low speed, typically with load reversal or at least load variance, and no issues at speed. I looked underneath while the wife switched from reverse to drive but saw no relative motion between diff and subframe, nor between subframe and body. Also thankfully no sign of floor failure. From underneath it also sounded a bit metallic, whereas it sounds more rubber-thunky from inside the car. Me thinks that the list of possible causes, in order of declining probablity, is ... diff to subframe bushings, subframe to body bushings, rear U joint, internal diff problem. So my questions is ... what is the most likely culprit and do I have to drop the whole rear package to swap the diff-to-subframe bushings or can they somehow be done in place?
Maybe a broken coil spring? This is pretty common. Check the coil spring closely, it's usually the end where the coils are smaller.
I did find a broken coil spring, changed the pair, with no difference. I'm certain it is the differential clunking, and related to drive torque. I'm just trying to avoid tearing it all to bits to find the exact problem but that may end up being what happens. Afterall, the rubber bushings are 10 years old now ... bummer.
The most likely culprit is the rear diff bushing in the rear that lies perpendicular to the diff. The diff does have to be removed to allow sufficient access to move the bushing.
My 01 325I auto with 106k miles had a similar rear clunk for a long time. It happens on a downshift from 3rd to 2nd while braking (mostly at moderate braking and less with light or hard braking). My shop changed the standard diff fluid with a heavier fluid. Will need to look up what exactly they used but the clunk is now mostly gone. I still get it but very infrequently. Just fyi in case you want to try that.
