I have this, or had this should I say, horrible rattleling underneath my car, like a nut or something was getting ready to come off and kick my ass. Driving around, especially at low idle, was the worst. It sounded like a ten dollar roll of quarters in my exhaust. And just having the rebuild done, less that 3300 miles ago, I was thinking the worst.
I torqued down every bolt on the exhaust, and still the rattleling occured. So I did what ever person does when they are this stressed, i pounded on the exhaust. I hit it with the heel of my hand, starting at the car and working my way towards the block. When I got to my cat, it jingled. I hit it again and it jingled again.
Eureka!
So then I took the cat off, and a parts of the stuffing for that cat started falling out, along with a jingling noise which turned out to be like a metal coupling inside. So I beat all the rest of the stuffing out, which there wasn't much left thanks to mister jingle-pants, and set up the coupling at the beginning of the cat, and hammered into the inlet tube, so as not to cause any more noise.

And now its quiet

 

hope you were wearing a respirator and some protective clothing. catalyst is a real bitch and can cause all kinds of bad ju-ju...it's toxic.

 

well after licking my fingers clean, I did take a bath....

 

hahaha!

 

classic.

 

the cat goes bad like that from too much fuel being ran through it. hopefully that was because of your last motor.