Just wondering if Roger E. ever resolved his "piston slap" sound. My 02 C320 exhibits the same behavior, however, it only does it when warm. Cold startups are quiet. If you look at what a piston slap is, this doesn't quite add up. Usually the piston will "slap" when the "skirt" of the piston is at its smallest diameter in relation to the cylinder dia. On cars with iron blocks, this usually meant slap occurred on cold start. With aluminum cylinders, this supposition may go out the window. I have another WAG (Wild Ass Guess). Could it have to do with a timing advance by the computer at startup where when the engine is cold, and there is extra fuel it's OK, and when the engine is hot you are hearing more of a "knock" or detonation? Just a guess, and it doesn't jive with the folks whose engine does it hot or cold...