Grinding, " diesel " noise under valve covers

Sometimes engines with high cylinder pressure can sound like that, especially when they're cold. The pistons will "rock" a little in the cylinders. It's usually more prominent when cold and as they warm up it slacks off some. I had a Chevy 350 that was already .060 over when I got the block. Had like .008" wear on it, but I broke the glaze with a .......you guessed it dingleberry hone and ran chit out of it. It had piston slap so bad when it was cold I got accused of having a diesel in it. lol But it had those old TRW forged pistons in it so I wasn't worried about breaking a skirt. That little bastard ran like scalded dawg too.