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.
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Exterior- stock painted black wheels, clear corners, stainless steel grill, cross and drilled front and rear rotors with evolution brake pads.
Coming soon- 18 inch black mustang bullet rims, GT/GTP rear bumper, tinted windows.