Rattle Noise, Seafoam made it worse then better.

I had a rattle in my engine at low RPMs under load for a few months. Happens when I'm going down the road and go up small hills. It'll make a slight rattle before shifting.

I did my oil change yesterday and did a Seafoam treatment as well. Put 6oz in the oil, ran it at idle for about 15 minutes. Did the aerosol in the intake as well, full can. Followed the instructions to the T. Changed the oil after.

So while driving it around to burn off the aerosol, pre-oil change, that rattle was much louder. Happened more often, especially accelerating hard. After driving around a bit it quieted down, and after the oil change it quieted down more to less than before the Seafoam.

So, question is, what the heck is rattling? I had thought it was possibly the catalytic converters going bad, not letting enough exhaust through. The Seafoam making it louder though, I dunno, maybe. Any other ideas?