In my opinion, your brake pedal has loosened up and is now rattling a little bit. Note that there is no noise when applying brake. Here is the test, on a deserted local road go over an imperfection that causes the rattle. Then with your left foot pull up on the brake pedal by wedging it behind and pulling towards you (a fun maneuver) and go over the same imperfection. Voila! If no noise time for the dealer to remove the play. Or maybe they will just break something else (I am sorry, but that had been my experience until recently). By the way this has happened in both my G's. Very annoying.