Do a search on this... there are a ton of discussions regarding this very noise.
Apparently it is a normal drive-shaft noise at low speeds and is a direct result of having the M-variable differential.
HOWEVER... If it has become something louder/worse/different after getting your 1200 mile service, I would want my dealer to have a look at it and make sure they used the correct fluids in the diff. For other people the noise is present from the first drive, so something new at 1200 miles would be a concern to me.