[Greg Reimer 7376]

Sounds like an object on a piston. A spun rod bearing could do that, because the increased bearing clearance allows the piston to travel up the cylinder on the exhaust stroke striking the head. That's a pretty obnoxious clatter, but the engine seems to run on all six cylinders. If a screw or washer or even a big piece of carbon landed on the piston and partially embedded itself, that would do it, too. Good luck!!