This could either be the valve train tapping until the motor warms, or the pistons slapping against the cylinders for the same reason.
This happens as motors get old. (High miles) There's nothing you can do about that except rebuild the engine.
And using oil additives is a waste of money. (Snake oil in a bottle) Don't bother.
The reason this happens is because the metal parts have worn normally over time. And when the engine is could, the metal components have a slightly larger clearance with each other. When the engine warms up, the metal expands and these clearances tighten up to close to normal and the tapping goes away.
And don't bother rebuilding part of your engine unless you're going to do it all.