Since your mechanic found no problems in the rear, perhaps the noise does not originate from the rear end. Sometime's, due to the way that sound travels through the chassis, one can be misled. Back in the 60's there was a guy who had noise coming from passenger side rear end, but no problem was found there. My Dad told them to check the left front wheel bearing. Sure enough, that was the problem. I certainly don't know if your mechanic missed something. I'd first check U-Joints and/or carrier bearing (if your car has a two piece drive shaft), then start checking the front end.