did you install new gaskets when you installed your new headers? where the header and cat connects at. also check if your exhaust pipe is not loose. see if its on the plastic holding thing, if its not on there right it will shake and might cause it to vibrate against something and make the rattling noise. the rattling noise can be from your muffler too. next time when you turn your car on make sure its from your cat and not the muffler. if its none of those take the cat off(6 bolts) and shake it or hit it. if that is the noise you will hear it when you hit it. thats all i can think of right now. g/l