'91 740T sedan, 115k. My car is making a whirring or whining sound when in gear. It increases in volume when pulling hard, or when the tranny is holding it back going down a hill. And it changes pitch according to RPM. My brother told me that the change in pitch indicates that it is not any of the universals or the center bearing, as sound from those parts would be constant. His guess is that it is in the differential. I bought the car used about a month ago, and was aware of the noise at the time. This was a one owner car with extensive service records, although I did not find any records relating specifically to the driveshaft, motor mounts, diff ect... I have inspected the motor mounts as well as the rear transmission mount and all appear to be fine. Any suggestions are appreciated.