[KingLT1]

Could be either one or both. They are known to rattle. There is a epoxy fix for the NPP valves but it seems hit or miss. Best bet is to buy a X pipe, axle back, and replace the stock stuff. Factory NPP sounds good until you start changing stuff with it, then it tends to get raspy and rattles with any change in exhaust flow. Borla makes a X pipe with resonators that will help keep the sound under control if you have cat less headers.