Are you running forged pistons? They need more bore clearance especially race forge, and clatter most when the engine is cold.
Does it sound like the noise is from one place only, as in one clack per firing of only one cylinder, or does it sound like all cylinders are clattery?
If it sounds like only one cylinder, you could try pulling each spark lead off in turn. If it's piston slap, the noise will disappear when you stop that cylinder from firing
That it happens at idle tends to rule out fuel/combustion ping
All of your points are typical of piston slap, except No 4, though it may be because of extra heating of the pistons making them expand a bit more and run quieter.