I've got this annoying rattle coming from underneath my car. It only happens between 2k and 3k rpms. It increases with rpms, somewhat. I thought it was a loose bolt on the exhaust, but I have checked most of them. It seems to be coming from the cat. Any ideas? Could it be coming from inside the cat?

 

Most likely it's your heat shield over the cat or the guts of your cat are starting to come loose inside. Smack it with a rubber mallet and see if it's rattling or wedge something under the heat shield and see if it goes away.

 

it could be the i-pipe vibrating on the cross-member(i think thats what you call it). Mine does it now until I can get it welded in place.