What owners with a rattle need to do is to put the car on a lift with it running and idling while the rattle is present. Stand under the car and see what is touching something it's not supposed to touch. That's what is rattling. You'll be able to pinpoint what it is.

 

The actual Cat is NOT the rattle. The only thing that can be rattling under the car are the heat shields.

The cats don't touch/rub on anything, they're perfect fitment.

 

Found the fix and I boy do I feel dumb. It was my r2c intake, the passenger side filter bolt came loose and the bolt was rattling against the heat shield for the filter.

Guess this bolt must have come loose on the dyno after the hfc install. Now I can go back to enjoying the car and not dreading that rattle.