Downstream is the resonator. Ignore it for now.
Whether the busted up cat (core broken loose) is the rattle you hear or not, it'* a problem. Replace it and let us know if it cures the problem.
Both of the cars in my sig had cats fail. Both of them made me think I had a problem with the engine or the trans. Yes, the second one fooled me even after learning with the first, because it wouldn't rattle side to side, but a couple weeks later, I backed it up on ramps and shook it up and down.
There'* at least 3 different 'sound and rattle' modes the cat can get you with. You can get a replacement hi-flow cat from www.justmagnaflow.com
(measure your existing in and out) and have a shop pop it in there for another 20 or so.