Haven't gotten underneath the car yet, until I get some crummy clothes on when I go home from school.

I actually had my top cat plate rattling with my magnaflow cats (not the pair that is on the downpipe) and it was just a tiny slight gap that with the best of my ability, I clamped together with needlenose and the noise stopped. The thing is though, when I had that problem, the cats would not rattle when the car was warm, but would BUZZ when I was going up in the RPMs on a cold engine... Isn't that odd?

 

my cars doing a similar thing, but its due to a worn out tranny. a couple bad bearings. its not a rattle, more like a soft grinding noise and it doesn't go away when i push down the clutch. so it might be a wearing tranny

 

yeah one of my passenger side cats is "buzzing" right now but its getting worse. and i believe it to be a cat issue that your having right now, but you should get under there and check before you make a purchase. although i think you have the correct diagnosis. one of my cats on my jeep ate **** and its rattling like hell

 

Well, now I realize that this sound comes on lightly even when I first start up the car after 2 days of sitting in 40 degree weather. It almost sounds like a knock, but it is most definitely more of a rattle. Just the consistency of the noise could be described as a knocking. Any more guesses?

 

Can you post a sound clip of what noise?

 

^^^That would be my guess^^^. Also on my `00GT, there are 2 electrical connectors snapped into a plate right under the trans. I`m guessing they are for the 02 sensors. One of mine came loose, & the connector rattled like a bitch on the plate. I used a zip tie, & all is quiet (sort of) again.