what is the catalytic converter and how do you know when its going bad

A catalytic converter is a vehicle emissions control device which converts toxic byproducts of combustion in the exhaust of an internal combustion engine to less toxic substances by way of catalysed chemical reactions. Yours should still be good. You will get a CEL if the downstream O2 sensor detects a out of range condition. --- You just might need the exhaust system replaced.

Agreed. Cats don't usually make an engine louder, they tend to plug up an exhaust system and set a Check Engine light. A cracked exhaust manifold (rare) or a damaged/rotted muffler is more likely the cause. A lot of muffler shops will check it out for free or nearly free. Go to a couple and see what they say.