No special knock that I’ve noticed. What I have noticed, though, is that those who do knock always open the storm door (double-pane glass insert in the winter; screen in the summer) to knock directly on the door.
Which puts the average male or female directly at eye level with my conspicuous-as-all-get-out “NO SOLICITORS” sign.
I guess they either get lazy and stop noticing, or they figure that I’m definitely going to be the one guy whose “NO SOLICITORS” sign … really doesn’t mean it 