Phantom Knock
Old trw forgies will rock/knock till they heat up enough and expand they'd run 8 thou skirt clearance.
On a side note my KB hypers are set on the loose for RPM/drag race and you can kind of hear'em when it's cold.
Sometimes you can scuff a piston skirt mildly, have it knock but then go away when warmed up.JMO
Glad you found the knock.