Every trade has an inside joke; among Hollywood sound engineers it's the Wilhelm Scream

Dan Seitz explains:

     A dude screamed into a microphone about 60 years ago and it's been turning up in movies ever since. It was originally recorded for a movie called Distant Drums in 1951 as a series of "pained screams" which were recycled for a few movies, and then lost until about 20 years later, when Ben Burtt found the scream on a reel labeled "Man being eaten by alligator" and stuck it in Star Wars, as the sound of a Storm Trooper falling off a ledge.
      Burtt included it in every Star Wars and Indiana Jones movie, and it's been a running in-joke with sound designers ever since, showing up in everything from Poltergeist to Pirates of the Carribean...

    At least the provenance of the above is well-known. Another Hollywood audio meme is not: “liberty 285, code 6, 105 North Avenue.”