1998 Chevy camaro crunching noise

Can any one tell me why when I start up my camaro it sounds like fork in the garage disposel I think its coming from the muffler not sure.I change the oil because it was is black it was disgusting.after that it sound a little better but still rattles any advice

Rattle in the exhaust could be the catalyst broken up in the converter.......hit it with a rubber mallet to see if that is where the noise is coming from...

It could also be an exhaust hanger weld that has broke. So the metal from the hanger is hitting the exhaust system at the broken weld point.