I vote for piston slap. From what I see this was a re-ring in a somewhat tapered bore. An old school refresh that works but sometimes is a little noisy. Many in the old days would intentionally put engines together loose in this fashion to reduce friction and haul A$$! They do make a little rattle at idle though. Many engines with forged pistons will also make the sound when cold because of the extra clearance needed for the different expansion ratio of the forged material. If the pistons are of factory design and were put in backwards (which was also a common speed secret) the slap is worse. The indicator will be the location of the "notch" that usually goes to the front of the engine will be at the back.