[Steverino]

I agree with the ideas of checking for lose items under the cargo floor as well as checking the hatch tightness. Sitting in the rear while someone else drives is good too. Could even by a lose muffler caused by a bad hanger or possibly a bad strut. There are only so many things that could go bang when going over a bump.

Otherwise, the dealer will follow the GM process. It will either address the issue or it won't. If it doesn't they will look for another culprit.