I know when you open and shut the doors the windows will move down about a 1/4 inch and then go back up after closed. it is similar to that noise but the windows don't move.
Nithros described what mine does, I don't usually have the key in my pocket and I don't have the automatic unlock feature. of course I am unaware if it does it when I am not in the garage.... Sort of like the sound of a tree falling in the woods with no one around.