overKILL

Number one,... what is that canister right after the exhaust flanges and before the CAT? I have this rattling around 2250 and 2750 rpms and its really starting to bug the shizz out of me. I checked for heat shields and they are all on tight. When I bang on the exhaust there is a buzz like rattle coming from the canister. What is it? Also,... I just replaced the plugs in my truck with Bosch platinum +4's,... and I can't decide whether they are doing anything for me. The car I owned before this truck,... a porsche 944 ran a hell of alot better with regular copper single tip,... I dont know whether it's just me or what? Which plugs do you guyz perfer?