ya sounds like a cat problem to me. if you car is running like crap the cat is probably plugged or getting that way. and gm a lot of the time would put a cheep heat sink like thing on em to try and keep the heat way form the floor of you car. some times they come loose especially with like the 2500 pickups. this is gonna sound dumb but since it is just rattling just hit the plate with a hammer till it quits rattling and then replace your cat when you get a chance. that will at least make it so the rattling part doesn't completely annoy you.
