Knocking sounds refer to the sound of something hitting a surface, such as a door or window. Raps, which is a 4-letter word, is a type of knocking sound that is usually made with a quick succession of short, sharp knocks. Clacks, which is a 6-letter word, is another type of knocking sound that is usually made with a single, hard knock. Both raps and clacks are commonly used to indicate someone's presence at a door or window.