I was thinking maybe a wrist pin also gm. I just can't see it coming and going like that. I think you may have something with the RPM gm. monty509, does it always go away at higher RPM? If it is a wrist pin, I would assume it has damaged the cylinder wall by now. Compression in that cylinder would be low. The wrist pin is clamped in pretty securely with a bolt through the rod. It certainly could come loose. I wouldn't expect it to last long. It is so loud. I wouldn't run it like that. I think a compression check is in order. If a cylinder is damaged that should show, or if a cylinder has a carbon load compression will be high.