So I have an 02 mustang gt that I recently discovered had NPI heads so I put some PI heads back on. But afterwards I’ve noticed a light tapping/ticking at idle it seems only once it’s warmed up. If I go end the engine bay I can’t hear it whatsoever, but only under the car, I’m thinking it is an exhuast leak. It seems to increase with rpm but then completely goes away after a certain point. It runs great and when I’m driving I can’t hear it at all. Does anyone have any ideas?