To save you time: I know the word rap and knock have very many meanings, but I only refer to the sound made at the door to ask permission to enter.
I'd like to know the difference between the two words, especially in terms of sound and strength. Is one stronger or louder than the other? Does one sound more polite and gentle? Is there a difference in register?
Knocking (n): a sharp rapping with the knuckles, a hard object, etc, esp to capture attention the sound that an engine or bearing makes as a result of wear or faulty combustion.
To knock (same link as before): To strike a sharp audible blow or series of blows, as on a door.
To rap: to strike (a fist, stick, etc) against (something) with a sharp quick blow; knock.