Which is also what detonation and pre-ignition sound like. Piston slap in our engines is a generally non issue.

All of our engines should have a metallic "tic" sound when the crank passes through top and bottom dead center. I would be concerned about one that did not. Typically that sound will not be heard when the engine is running. If an engine has a distinct mechanical clanking sound when running it should go in for a check up. The problem is, so many people do not understand how to at least reasonably tune an engine they don't know how to differentiate sounds that tell them what they need to know. Tuned too lean with a low grade gas on a hot day and they get detonation. Running a carbon dirty engine permits accumulated carbon on the spark plug and combustion chamber to become hot enough to ignite the fuel charge before the normal ignition point (pre-ignition) and they have a similar sound. Both can and will severely damage or destroy an engine if left uncorrected.

If you've had a couple of prop strikes and mechanical sounds are more than they used to be, send it in for an inspection. The engines were not designed to handle the severe stresses of prop strikes. Bearings fail and cranks bend. At the level one should be at by the time they get to gas engines their ability level should be high enough that prop strikes were left behind with .40 size glow engines. Bearings cannot take the abuse that bushings did.