Well the date with the mechanic has been pushed till Monday, and I drove around a bit to be more specific.
It's definitely a bit "rough" on acceleration. I can feel a vibration on the accelerator and the steering wheel. When I let off the gas at 50mph and up, the noise (sounds like your driving on the ribbed side of a highway) chimes in for a few seconds, then dies down as the engine slows down.
My mechanic says he wants to look at the pinion bearings in the differential.
Does this sound right? Any idea of what cost I may be looking at? He'll be checking the lube as well.