Aftermarket cat dying?

[99W68Firebird]

It started rattling recently and i dunno if a bad cat would make your exhaust sound like its leaking but when im next to a building or curb and accelerating a lil bit it sounds like theres air coming out from under the car.

Also i have no idea what kind of cat this is anyways, its 3" in/out aftermarket but never had a brand name on it. But here is a pic of it if anyone can help identify it. I think it looked like the magnaflow one, but doubt its magnaflow, for all i know it could be Murrays auto store special lol

Pic of the Cat

Also if this thing is shot should i buy another cat? What brand? or since i have o2 sim in there now jus put in a straight pipe cuz theres no emission tests in michigan, but im wondering how that would sound and if would hurt the back pressure.

[98Camaro3.8]

First thought would be a heat shield coming loose. Tap on it with a hammer or something and see if it rattles. If it does, try just removing the heat shield around the cat.

[99W68Firebird]

heat shield around the cat was removed 2 months after i got it LOL so already been thru that

[BiH]

I would just put a straight pipe in there ;)