Hey guys, I got 2 new catalitic converters a couple months ago, and one started rattling. I had it replaced, and low and behold the Cat is rattling after less than a week. The guys at the muffler shop said that it was probably due to heat damage, but my local car guys says that would only be caused by something that would make the check engine light come on. I have a 1991 z28 with headers and a flowmaster. Any clues?

 

Don't know if this will help any, but it's worth a try.
At one time I thought the cat on my ride was rattling when I turned the music up loud. But when I got up under there and checked things out, it ended up being my heat shield right above the cat.
You didn't mention if yours was rattling from exhaust gases when you punched the gas or just from vibrations driving down the road, so i don't know if this will help any.If it is only rattling during acceleration, then obviously it wouldn't be the heat shield.
Anyway, just thought I'd mention this just in case.

------------------
"So,tell me Mr. Stang. How's life like living in the exhaust of a Chevy V-8?"