BMW Differential vs. Wheel Bearing Noise
This answer is applicable for many BMWs in addition to the one listed below.
car year: 1999
car model: 323is
My Bimmer started to make this whining/puddering sound, almost sounds like a wheel bearing. The noise increases with rpm and seems to get louder and more noticeable under heavy load, i.e downshifting and letting the gear slow down the vehicle. I had a driveshaft center support bearing installed, but nothing changed. I did notice some small metal flakes in the rear differential fluid so I changed the oil, still makes the sound. Do you think this could be a bad differential? Any other ideas would be greatly appreciated!
The way you’ve described the noise that you’re experiencing makes us think that it is from the differential. Either bearing noise or gear noise. A differential will make a whirring, whining or growling noise if one of the bearings is worn or the ring and pinion gears are worn or meshing improperly. Similar to a wheel bearing, the noise will change with vehicle speed. However, a wheel bearing noise will not change with slight changes in load (slight throttle on/off changes, coasting versus accelerating, etc.). Additionally, a differential noise will typically not change as you turn right or left (loading the vehicle to one side or the other), as a wheel bearing can do. Note that this only applies to independent suspension designs. A “live axle” design (such as many domestic rear-wheel drive applications) could also change it’s noise when side loaded, if the gears or bearings are faulty.
In reality, a bad differential bearing or gear set is not really something that you would repair yourself. This requires that the differential either be rebuilt or replaced. A used replacement unit is likely your lowest cost alternative.
In the case of a failing wheel bearing, Bavarian Autosport stocks the bearings and the tool required to perform the bearing replacement. See the link below for a blog post containing additional information on BMW rear wheel bearings.
We also have a complete DIY on replacing rear wheel bearings, using the B90-BMW wheel bearing tool.