1st, put your floor jack under the diff and raise until the wheels are off the ground. Check the rear coils. Are they tight or have they become loose? If the coil or coils are loose when lifted, the OEM rubber strut mount bush has pushed through the upper strut plate. This will make a lot of clatter. Generally it occurs when a top or bottom washer to the upper mount plate has gone awol. The fix is to press it back it and reinstall with the top and bottom washer.
I hate it when parts decide they would rather be on the floor or bench or rattling around in the trunk.
2nd, Chevrolet made a change to the rear strut upper areas creating a dome like space. They did this because the washers and top strut not would occasionally make noise under contact. We have seen this on a few cars. The fix is to take an air hammer with a nylon bit and create a dome by banging away. This occasionally happens to the OE configuration and occasionally with a Pedders install. You my need a bump in thr trunk.
Finally did you have a friend helping you with a dark sense of humor? Check for steel marbles. Before you say that would never happen, I should admit... the weather in Indy looks great for this weekend.