Overheating Piston slap might be an occurrence with a badly worn car engine, where the wear on the cylinders and piston skirts results in excessive clearance or play. The evidence of this would be noticeable at low or idling rpm.
As the pistons travel up and down, the reaction force of the connecting rods will impart a slight sideways motion on the pistons. The resulting motion makes the piston skirts “slap” against the cylinder walls and emit a clunking sound.
The phenomenon may be somewhat reduced as the motor is accelerated, however, it is an indicator to the general mechanical state of the engine. The solution to correct this would be to have the cylinders re-bored and larger pistons (if available) fitted in.
However, at this point the owner of the vehicle/car should decide if it is worthwhile to get the engine rebuilt if the rest of the car is of sufficient worth.