knocked

A knock is a sharp sound made when two hard objects hit each other.

When someone says "knock on the door," it means to make a gentle tapping noise with your hand on the door, usually to let people inside know you're there. You might knock on a friend's door before coming in, or on a door that says "do not disturb" if you need permission to enter. Some people also use their knuckles for a louder, more insistent sound.