I never noticed until last night after I was just cruising around breaking in my 3.73's, but every time I tromp on it, I hear what sounds like a metallic type noise not a pinging or knocking or anything engine related, but coming from the my mac o/r pipe.. it sounds perhaps like it's nearly right below the dash, where the exhaust manifolds bolt up to it.. when I first did the swap I heard the same noise, but it didn't appear to be a leak. I'm curious, do you guys who have this pipe all hear this noise under acceleration, or do you hear just straight exhaust and no other sound?