[darrowco]

I vote for checking the piston/head clearance. I had a similar knock and traced it down using an accelerometer and an oscilloscope - the knock lined up perfectly with the exhaust stroke of #3. At that point, I realized that the recently skimmed head had gotten one skim too many. The engine turned over freely by hand - the knock only occurred when the piston was flung up while running and took up the slack. Once apart (for the 3rd time) the mark on the head was beeeaaarly visible. A little relief cutting in the head solved the problem.