My girl friends drives a 94 Camaro, and when its in drive it rattles, and we thought it was the tail pipe, so eing a red blooded Kentuckian I used wire to tighten it...well that worked, but it still makes the sound, so to day I crawled up and she turned it on and it turns out its part of the exhaust system, but not sure what part...anyone heard of this before....its right under the passanger seat!

 

Good chance the inside of the cat disintegrated and rattling inside of the housing. I looks like a muffler and live right under the bump in the passenger side floorboard.