There is this weird rattle or knock that comes from the cat or alternator?

Ok so, I have a camaro, 2000, 3.8L. I have a code that I got from the check engine light that says Catalyst System Efficiency Below Threshold (Bank 1) or something like that. I am confused if it is coming from the alt or cat? If so, what is the best and cheapest way to check if the cat is bad? The car has 115,000 miles on her. The knocking sound is really pronounced on cold days but goes away either after seconds or after the car warms up.