Rattaling Lesabre

first time on this so bear with me , I have a 2004 buick lesabre custom 3.8 . and when parked and running it sounds like a sword fight is going on under the car. first i thought it was a heat shield on the exhaust somewhere but they are all tight. When i had my tires rotated the mechanic came over and ask if i knew the catalytic converter was bad and he said they rattle that way when they are bad or going bad . true or not ? I don't know but its getting worse now and drives us crazy . please any help would be most appreciated .

No way to diagnose rattles from here, but try tapping the cat with a rubber hammer, if it is broken inside you will hear it, mufflers can act this way also.

Thanks , it does .

My son's LeSabre was also making that noise. We didn't take any corrective action (because we didn't realize the source) until the catalytic clogged and the back pressure caused problems. Replacing the cat corrected.

Then he wrecked it.