I have/had this problem on my car and it was the heat shield on the driver side. If you go under the car and try to move the heat shield side-to-side, it should not budge. On my car, it did not budge on the passenger side but it moves on the driver side. Kinda hard to tell if it was a spot weld that came loose but I couldn't see what else it would be.
I came up with a hack solution, but I did notice there was some white insulation-like material around parts of the exhaust. I wonder if you can get some of that insulation and stuff it between the heat shield and the cat to get rid of the rattling. I don't know what that white stuff is, and I don't know if stuffing it in there would be a good idea. But if it's ok seems to me that would fix the problem in a manner better than what I did. It was hackish enough that I don't even want to disclose it .

BTW the dealer "solution" to this is to install a new cat at over $1100. Unbelievable. They simply don't fix anything, just replace it with all new parts. Sometimes that makes sense, most times it does not.