Originally Posted by automch

A gear whine can run for thousands of miles without being a problem. If the ring and pinion were not set absolutely correct, if at some time the lube got low, or if a bearing failed on the pinion or carrier, a pattern could have been worn into the gearset that will cause a whine. It should be noted that ring and pinion whine reaches a resonant speed in the 35 to 50 mph range depending on ratio. In most cases it will not cause an issue unless it becomes pronounced. But are you sure it is differential? Most ring and pinion whines will increase with light throttle pressure and many will dissappear on coast. Or sometimes the opposite. If the noise is constant and independant of throttle load, I would suspect center bearing or pinion bearing, or axle bearing, but these usually growl, not whine. HTH and Good Luck.