removing the resonator is GHEY! That's the reason it rattles like that. Watch, here's mine with headers, 3" dp, thrush glasspack, and 40's mufflers
HPIM4007.flv video by ninnyGTP - Photobucket
*edit* the reason they sound different is Firebirds/Camaros only use a single muffler and that GP had it's resonator removed
Just for comparison, here's my car before and after the glasspack
http://www.youtube.com/user/ninnyGTP#p/u/1/-tCprMiwnww