I've got an '08 CTS, and I hearing a thumping noise coming from the passenger rear when I hit a manhole cover or bump in the road. It's driving me crazy! I took it to my mechanic and he tried everything to find it. He took the sway bar off & that wasn't it, then he took the strut off and checked it, that wasn't it either. We checked the trunk, the battery, the exhaust & everything else we can thing of and it's still making the thumping noise (He gave up). Has anyone ever had this problem?