Weird marbles in a can?
 

So I recently heard the marbles in a can sound but it sounded much louder than any of the videos I've watched on it. Now instead of the marbles in a can sound it gives off a "hollow" sound with what feels like a loss in power too. Happens around 4500+ RPMs. Also, I went from a typical 18/19 MPG to 15 last time I filled up if that means anything. Just wondering if it's the same problem or something else? Feel like it might be my catalytic converter. It also seems to only happen when in gear.

are you throwing a CEL? does your exhaust smell horrible?

Smells bad but no CEL. I did test drive an rx8 with a bad cat that didn't given a CEL but had a horrible smell. Smells like that other 8 but not as strong so I don't know if that's normal.