Cylinder Bore Clearance

This is a problem with older high mileage engines. The clearance between the piston and the cylinder wall increases leaving excessive clearance between the pistons and their bores. The pistons are typically made of aluminum. When they’re cold, they are at their smallest. As the metal heats up, it expands and fits tighter in the cylinder bore. The clearance between the piston and the cylinder walls has decreased along with the noise. The knocking sound is known as "Piston Slap".

Piston slap

Most pistons are oval shaped, with more diameter around the thrust side than around the piston pin side. As the engine warms, the piston expands and fills in the piston pin side, making it round. These are called cam ground pistons. If there's a slapping noise in the engine that goes away as it warms, run a wet compression test to see if the cylinders seal are within specifications. Always check for TSB's and for any manufacturer's special procedures.