Hmmm....how did you determine that the keyway was worn? Need to get a better handle on when the clatter occurs and disappears.
valves make more of a ticking noise, this was a clack. it went away at higher rpm instead of getting worse. was independent of hot or cold engine. but it was when i removed the cam tower and had assy in my hand that i felt it.
originally i thought maybe a belt hitting a loose bolt. anythings possible.