Car only has 54K miles but at speeds up to 70mph there's a high frequency whirring sound at constant acceleration coming from the back end. Above 70, I don't hear it. Other than that, car is a dream.

 

Decided to leave the Jag with my mechanic and I'll know more next week.
But no, when I had the oil changed, we inspected the rear end and saw no leaks. In the meantime, I contacted the dealer who serviced the car since new before I bought it used and the Service Mgr said if it's gear lashing in the diff to drain and replace the oil with GM posi-traction oil. Said it might deaden it or even eliminate it all together. Worth a try I guess. Ever hear of anything like that?