Do you know if your chain tensioner is the older, oil pump actuated one or the newer spring loaded version? Motors that had the original pump actuated tensioners would have a a tell-tale metallic rattle at start up, which would diminish or disappear when oil pressure built up. With the spring tensioners, there was no need for oil pressure to activate the tensioner.

It could also be that the metallic "slap" . . . if not isolated to the timing chain cover area, might be signaling a stuck or sticky valve lifter.