I have a 1989 951 with a bursche cat bypass and stock exhaust. My car has a nice low rumble to it when idling around town but my friends (whom I have passed on the highway several times) tell me that when I am under boost my exhaust loses some of it's rumble and gets loud and tinny sounding. Inside the car everything sounds good and I don't notice any tinny noise. Could this be the result of the cat bypass? Would a different exhaust/muffler fix this buzzy/tinny sound? From what my friends describe I almost thought I had a broken baffle in the muffler.

 

Soloracer -

check the gaskets on the wastegate... They might be leaking after the gate into the test pipe.

HTH!'

 

Check the tin heat shield over the area where the cat is /was, the brackets have a habit of breaking.