Could be the one of the rear wheel bearings or simply a tire rubbing on the wheelwell, but that should only happen with oversized rubber, which you don't mention having.
The 'machine-gun' sound sounds like the driveshaft possibly touching the floorpan.
Try to get the car on a lift that supports the four wheels and put two people in the rear seat. Then see if anything touches anything. Best I can do from a distance.
