1998 Honda Civic LX Noise

Hello everybody! I have a rattling noise coming from under the car. I looked under the car and the exhaust seems to be fine. I open up the hood and it seems to me is coming from the catalytic converter. The Sound is louder when the engine is cold and diminishes when is warmed up. It does it when I am at a stop light or when the engine is idling. The car has 142k. Is it worth fixing it? How much would it cost if that is indeed the problem? Any suggestions? Thanks in advanced!