@hobbs also the number of knocks is an interesting variable you can play around with. for example:

1 knock: not even knocking. wont even notice
2 knocks: polite. might not think it's someone knocking
3 knocks: someone probably knocking on the door
4 knocks: its important
5 knocks: maybe even urgent
6: its gotta be the cops or some shit
10: it isnt someone knocking, its probably a neighbor putting up shelves
50: they must be really bad at putting up shelves
100: what is even going on out there