What causes a slapping noise coming from the piston area?

What causes a slapping noise coming from the piston area?
Bad things.. Worn cylinder, worn piston, worn rod bushing or bearing... or a combination of those.

But if you aren't good at diagnosing where sounds come from.. it could be a partially collapsed lifter or bad cam lobe too. Those are actually less serious.

Piston slap = connecting rod issue