Originally Posted by Tractionless

I developed the hiss which sounds like quick metallic rapping imo like a leaking exhaust gasket, immediately upon installing high flow cats. I brought it to my exhaust guy to verify none of the gaskets were leaking, they were not. Luckily mine only happens for a couple hundred rpms around 2000 rpm during decel.

From what I've read ,it culminates from the headers/cat area where the sound is actually produced and heard so it doesn't matter what else you have installed downstream as the sound is not coming from the tail pipe(s). Seems to be exhaust pulses hitting the exhaust piping walls in the component bolted the the header.