check your mounting/clamping brackets, make sure they are tight... my cat was make a sound like a coocoo clock, really anoying...so i just took it off... runs good...not an option if you live in a smog area tho...you could try to gut it out so it looks like you have a cat...or just buy a new one, cheap on is something like $100, i think.
A lot of the older cats were running "buckshot" through them for packing. It might have slipped some out and that's the rattle. You could replace them with a unit that runs better and makes more power (not very much power) with a monolithic cat from an aftermarket supplier.
I don't know how it is anywhere else, but in CA, you have to basically demonstrate a need to replace or it needs to be over 115,000 miles to replace it. I have no frickin' clue why, but that's what I understand.
In new york where i live the law states that you have to have a cat but it doesn't say it has to function. So i took mine off and knocked the porslon out of it and dumped it out. Doesn't rattle anymore and now is more free flowing.