2013 SE Left rear THUNK noise on take off.

I just bought a 2013 Edge SE with 100k miles. The car is beautiful. On takeoff fwd or rev there is a very LOUD THUNK sound that seems to be coming from the rear, left side. My mechanic has checked all bolts, etc and everything is tight. The rear end is brand new. We removed the drive shaft and the noise stopped. We have checked the PTU + the rear end. When my mechanic watched me take off, he said it looked like the rear end wanted to fold up in itself. Strange I know. Anyone with any ideas would be welcomed.