Possible rod knock?

Could also be flywheel to crankshaft bolts.
From the description of how it “comes and goes” it isn’t a rod or piston.

Lifter noise can come and go with temp changes, and change with an additive.
Rod and piston noises don’t come and go generally.
They would sound one way cold then change as the engine warms up. You wouldn’t be driving 40 miles without it making noise if you had a rod knock.

Believe it or not, even a belt can make a pretty loud knock type sound.