2000 Trans am WS6

:banghead: hey i have 00 trans am ws6 and i had some one put an exhaust on it a while back and its got a raspy niose to it when it gets to about 3k rpms that sounds pretty bad but if i shift before i hit that mark or when i just go past that point it sounds sweet but i was wondering if anyone new what was up with that any help would be awesome.
by the way could if possibly be my cat. because my check engine light is on and have ran a diagnostic and it says that it is sensing something in the catalytic area.

Your post has been moved from Pontiac Non-Specific/Classics to F Bodies Problem Diagnosis to help facilitate an answer.