My 5.7L makes a diesel-ish sound most noticeable while at a low idle such as when the vehicle is in gear at a stop. I have used a stethoscope around the valvetrain but nothing seems to fit the same sound/pattern. Some folks say it’s piston slap and it’s normal for this engine but it’s not as bad as the youtube videos show piston slap to be. Otherwise the truck runs great, just a bit annoying.
You are describing classic piston slap. If it is worse at cold startup and gets progressively quieter as the engine warms, that is a clear confirmation of piston slap. It doesn’t hurt anything, it is just annoying. It will get worse with wear.
It is a balance between tight piston clearance to quiet the engine or loose piston clearance to reduce friction for better mpg’s. Wear, obviously, walks the engine toward audible noise.
Thanks for your reply.