There is a rubber or plastic stop in the outer edge of the door that keeps
the glass from banging against the metal of the door frame.  It is above the
lock mechanism.  Mine fell off and the same annoying banging in the down
position.  Take the door panel partly off and you'll probably find the stop
inside the panel somewhere.  You can rig something up if not.   I found a
plastic stop and pushed it into the mounting hole and problem solved.