Capture the audio sounds of you knocking on a door, or a table, or some other material. Make it stranger than strange.
If it helps, crank the Rolling Stones Can’t You Hear Me Knocking on your headphones (but no you cannot use that song as a Daily Create, you gotta create it yerself)
(CC0 licensed audio by anagar found on Freesound)
Yes. We hear you. Knocking. Who are you?