Winston, can you provide a photo?

I installed all my exhaust components, and from what I remember about my cat/B-pipe connection:
- cat has a triangular plate welded to the output piping; my stock cat had three bolts attached, while my new cat had only three holes (which was a PITA; luckily I had three suitable bolts and nuts available)
- B-pipe has a matching triangular plate welded to the input piping, with three matching holes
- cat-back kit came with metal/cardboard-looking gasket
- I used copper hi-temp gasket maker

So I don't understand what is rattling? Is it something in the cat? Is it something in the B-pipe? Is it a nearby heat shield hitting the cat and/or B-pipe? Is it the bottom of your shifter hitting the B-pipe?