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Usually that means that the elements inside cracked and now if you tried to smog with that, you probably wont pass. I would find a decent used cat and put it on.
Look in the for sale forum...there are so many people who have test pipes, cats are relatively cheap. Or you can pick up a high flow cat...I think Berk makes one.
destroy the inside element with a piece of pipe and put it back, then you can use your car with no rattle sound on it. that give you time to buy another cat or a test pipe.