I've heard the piston slap when its cold (34 degrees in sunny ca) but my truck recently had a little louder tap at start up. I took it to a guy here that does performance LS motor stuff. we originally thought it was a lifter sticking. I couldn't get the noise at his shop cause the motor was warm. so he came to my work after it had set a while and was cool. turns out it was the cat takin a poop. since then its turned into more of a rattle... all the time! I need to get it ironed out but a replacement is near a grand. I have to get a CA legal unit. I hate CA. I hope its just piston slap in your case...