A little piston slap on late small blocks is not unheard of. I have seen a couple high mileage stock engines and have a vortec myself that were pretty loud on cold mornings and got quiet after a little warm up. Especially since you don’t know the specifics of the internals on your engine, that is entirely possible. The sound going away as it warms up is pretty common for piston slap, but definitely not exclusive to it. The mechanics stethoscope suggested by others should help you narrow it down. If you send off the oil sample you should get an idea of bearing health.