graoning/rumbling in top gear 83 300td

I've just managed to get the car back on the road. What I notice is that when the car shifts into top gear I hear a distinctive rumble or graon from the rar end of the car. The noise disappears if I lift off of the accelerator pedal and returns when I return.

My guess is wheel bearings, differential or top gear in the transmission - any suggestions as to how I track this one down.
Steve