Be sure to check the rear shock mounts, i've heard they're a common cause of this noise.
The mounts (old and new):
The shocks with the mounts on them (old/new):
My old ones were pretty bad but they hadn't begun making noises yet when i replaced them. Kinda easy to replace if you have a lift/jack+jackstands. The mounts are located under the carpet in the trunk on the sides on top of each wheel well. Bolted to the car with 2 nuts and to the shock with 1 nut+washers.
Edit: also if it grinds when accelerating, just look under the car at the diff housing and check if it leaks any fluid. The washers deteriorate with time and when i changed my diff fluid after buying the car there were less than 1mm left of the washer to prevent it from start leaking. I've changed diff fluid on numerous BMW's and those aluminium crush washers have always been close to start leaking.