The differential in my 1992 928 GTS has exhibited a slight whine after the
differential oil has warmed up. The symptoms are classic for a differential
pinion that, over time, has lost the factory pre-load settings.
The symptoms are :
1) The whine usually starts after the differential oil has warmed up
2) The whine changes or disappears with throttle variation. Either take load off, or increase make as it stop.
3) the whine is typically showing at 55-60 km/h. Not above or below.
Pinion pre-load is lost gradually over the life of the differential as the bearings and or race wears with use. This wear can cause enough pre-load to be lost that the pinion will actually move when torque is applied or removed to the pinion via the driveshaft under engine acceleration or deceleration.
This noise is a result of the pinion shaft moving slightly, thus changing it's angle to the ring gear, as on/off engine torque is applied to the differential. In most cases this condition is more of a nuisance than a sign of impending disaster. Further complicating matters, the fix is often worse than the problem. A complete teardown of the differential was necessary to get to, and replace the pinion crush sleeve which controlled the pinion pre-load.
This is exactly the symptom my car was exhibiting, a imperceptibly movement of the pinion under torque variation that produced a slight differential whine.
Some pics by Errka