[Kurt in NJ]

Another possibility is a lifter that has a groove, if it stopped rotating for a while and got a groove worn in it, then is now rotating every time it rotates to the groove you will have a noise , it will come and go at random intervals.

Piston slap usually gets quieter as the engine warms up.
Wristpin noises usually get louder as the engine warms up and the oil thins.

There is a KRW tool that would raise up the intake manifold so you could run the engine and look in the valve chamber for noises ---perhaps you could ask around to find one to use.