Frequently contactees report a knocking sound preceding their visitations, and almost invariably interpret this as a signal of some kind. Humans, of course, knock on doors, tap out Morse Code, beat on drums to send messages through the wilderness, and tap their feet or drum their fingers to express impatience. If they hear knocking, what else could it be? Density shifting is most often silent, with only the airspace disturbed. A ship suddenly appearing high in the sky, displacing a few molecules of thin air, creates at most a light change in the breeze. Large or sudden displacement of air can create sound, as thunder claps following a lighting flash attest. Arriving or maneuvering aliens, who do not arrive all at once lest they collide with each other at the staging point, create sudden movement in the air space in confined or delimited settings. Clap, clap, the aliens have arrived!