The rattling almost sounds like a heat shield. i did just buy the car and it is my first gm product so I am new to them but have had heat shield problems in some of my other vehicles. I will have to see when its in the air I guess. Does anybody know a way to tell if its the resonator or the cat? The resonator is only a year old and it is part of the magnaflow system installed. The cat who knows my guess is original.