Well this is going to sound stupid but I had the same problem to where I had a sound everytime I go over a bump. First off I don't have a GXP but I found out that it was my plastic case for the ONSTAR wires that it covers that makes the noise when I hit a speed bump, then I move that plastic case with my hand and it had the same noise. Problem solved but not fixed yet.

I dont know if you have onstar or not but I was just hoping this was a help to you but let me know if this was the problem or not.