The Hemi pistons are a short skirt, free-floating type and, as with most high performance engines, expand to ideal clearances when at operating temps to provide performance and efficiency. The colder it is, the more clearance there is so you hear "piston slap". There is nothing bad happening, just don't hammer on it when the engine is cold and drive mildly until you're at operating temp -- good procedure for any engine IMHO.
The Hemi rods and pistons are highly refined and lightweight which may also add to the relative cacophony on startup. Personally, I enjoy the mechanical noises the mild monster makes.