I used to have a cold knock on my stock motor. I started hearing it at about 40K miles and it was there right up until I changed headgaskets at 140K miles. After that it was gone and right up to now at 200K miles it has never made that noise again. My machinist said it might have been carbon. When carbon heats up it softens.
I always thought it was piston slap too but from what I've seen of SC motors and how they don't wear the cylinder walls badly, I'd say that the only way you'd get any slap out of a piston would be if a skirt was broken.