The Stovebolt Site Forums The Shop Area Engine Shop Knock on #1 cylinder

I know everyone will say that the combustion is causing the additional stress, BUT anyway: If it does not do it when plug pulled and it's still going up and down, it should be still making noise with some of those reasons given above. Just the change in direction should make it slap. So why don't you tighten up the exhaust manifold and check it for leaks and cracks (near no. 1) just to have something to do in your spare time...and use a broom handle or a real stethoscope to listen.
ALSO, Jerry can give you a test done thru the plug hole to test to see if anything moves just for fun. (rod knock test)