Maybe just pinging, but it could be your pre cats. Mine sort of sounded like BBs in a cofee can when they started crumbling. Put a stethescope on the exhaust manifold to see if that's where the sound is coming from. <lost-in-space robot voice>DANGER! Reving the engine with fractured pre-cats could cause a chunk of ceramic to get sucked into the combustion chamber.</lost-in-space robot voice>