Even a brand new cam can do it!!! Its not the cam its the lifters lose hydraulic pressure over night, and in this cold weather the oil is so thick they tap for a couple of seconds until they fill up with oil again.

Fair enough then if the tuners have said its piston slap but i cant think of a logical reason why it would do it for a couple of seconds on start up then stop, IMO its the top end, but thats just my opinion