There is a natural tendency for the piston to cock to one side near the bottom of the stroke. If there is too much clearance between the piston skirt and the cylinder, the piston will slap back and forth in the bore making a noise. All engines do it to some degree, but when it gets to the point where it is quite audible, you usually have a problem. Two-strokes are more susceptible. - Mark
As he said , just to add that its more common when engine is cold , when warm the slap should reduce. cheers
