A rattling noise coming from under car in the area of the catalytic converter can be the catalytic converter coming apart internally. Mine did that and while the car ran more or less o.k., it was a smidgeon down on power and depending on where and how it was being driven, it would run a tad hot. As the catalytic convert comes apart it starts to plug the exhaust system up. The repair is to remove it which will cause it to fail a smog check if your state requires you to smog check your car or replace it which is required in my state in order to pass smog. I don't remember the cost exactly but about $150.00 at a local muffler shop took care of it for me.
A pretty much generic catalytic converter was welded into place. While you can maybe obtain one from a junk car to re-use, in my state the local muffler shops will not install a used catalytic converter.

 

Good comments there, UnclJohn -- had the same problem with my '93SW ... new generic cat welded in, and good to go ........