Stopping the knock: Lower compression and ported heads...

Since there seems to be no physical signs of detonation, what else could it be? What else makes a knocking noise that goes away with higher octane fuel?
This one boggles the mind. Most people say my CR shouldn't knock, but I heard something. Now if the parts don't show any evidence of detonation, what the heck do I do now?

Piston slap.