More than likely it's SHO Shop's piece of trash cat interiors breaking up.
I've seen and heard of this happeneing to many people.
This is what I did:
~pulled off the y-pipe
~shook the cat and confirmed that the "guts" were breaking loose
~used a long handled screwdriver and broke up the rest
~poured out the "guts"
~re-installed y-pipe
Problem was solved.
