To me, it’s obvious that they’re the sound of a chair scraping across a floor, slowed down by an octave or so. Put that on top of video of the sky, or of people looking at the sky with concerned looks on their faces, and you get millions of eyeballs.
Steven Spielberg’s first movie was kind of like that. His senior year in high school, he went around with a super-8 camera. He went up to all the jocks and other people who had harassed him when he was younger, and asked them if they’d like to be in a movie he was making. Most said “yes.”
He then asked them to look up at the sky and - when he cued them - to suddenly flinch.
After he got ten or fifteen of these people to do this, he edited them all together, intercut with footage of seagulls relieving themselves in flight.
He showed it at the “senior assembly.” It was a big hit.
Speaking of which, I saw Spielberg’s”Super-8” on TV the other day — it is a real turd of a movie.