my car has this under car rattling. It rattles when that car is idle and rattles are around 2500 rpm. I checked all the heatshield, i even removed the shield from the 2nd cat, where it sound like its coming from. My roommate says that my 2nd cat is trashed, that the honeycombs metal is broken and bouncing around.

So how to do tell if your cat is broken? Also if i decide to cut the 2nd cat off and put a test pipe would it still pass emmissions?