my 07 has done it since i got it, i took it back when i got it, it gets worse when there is a bunch of people or weight in it?
i think its the lack of structure in the back. the dealer said the spare tire was loose and gave it back but it wasnt any different and i need to drive the car im not going to let them have it for a ton of time. there are no strut bars like my 03 had(it was solid and quiet for the almost 200k i had it) and everything on the car is just built thinner and creaks and groans all the time. ive been thinking of putting my own bars on it and see if that helps
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock