If you're really tight for space "Pls Knock" ("please" abbreviated) takes as little space as "Knocking." I would first be mystified and then get the giggles if I came across a door with a sign saying "Knocking," which implies that something or someone already is knocking.Knocking alone wouldn't mean much in my opinion. Knock please! would mean more. Knock all by itself is better than knocking but still a little odd.