Played Q*bert with my 5 years old yesterday and of course, he made his Q*bert fall out of the grid.
When the knockers knocked, my kid naturally said: "Q*bert fell at the bottom of your cab" and he started looking down.
BUT OF COURSE!
That sound is clearly supposed to emulate the sound of the little characters falling from the grid all the way down and hitting the bottom of your cabinet.
I bet if you open the backdoor right after a game, you can see a few Coilys and Q*bert knocked down lying next to the power supply.
After all these years I finally got it