I have a Flowmaster muffler with duel turn downs just behind the cab. I have a vibration that is new and driving me crazy. It was always a little noisy, but not like this. I took out my tool box in the back thinking it was that and it didn't stop. I can't seem to find anything loose, so I don't know if the cat converter is going or the muffler itself or what? Any suggestions would be much appreciated.