ok so this is my first time on here and i have a question i do have some basic knowledge on my car which is a 1991 pontiac firebird formula h/o 305 it is all original and the exhaust has started to rattle pretty bad it only does it when the car is in gear and at low rpm's does this sound like a bad cat converter to you???? this was my first assumption but not 110% sure

Yea sounds like the cat to me. The ceramics inside break up and rattle. Worse case they'll block the exhaust and cause all sorts of nasty problems. Just to be sure I'd crawl under and makes sure all the clamps are high and tight.

Oh and

The heat shield is notorious for coming loose and rattling too.

Matt