I'll do what no one else seems to have realised you can do: knock on the window. Makes just as much noise as knocking on the door and you circumvent the whole "anti-door knocking" law.
Or, I'll find some form of self propelled explosive device to launch at the door, as everyone who plays TF2 will testify, if you are not on the same team as the door, it is impervious to every kind of attack known to man, so should make a loud enough noise to alert whoever is inside, whilst not breaking the door knocking law. Though it may violate several others.
EDIT: Oh damn, ninja'd at the top of this page.