Tones When Pressing Keys

Intermittently, when I press some keys on the keyboard, I would hear a triple-tone from Windows 10. Nothing would happen, but the three tones have progressively lower pitch, immediately followed by a four-tone signal, which have progressively higher pitch. This is the same signal that Windows makes when one unplugs an USB device and plugs it back in.

I cannot figure out which keys trigger this reaction. I know for a fact that arrow keys do it, but I think the enter key does it also, or even the space bar.

This only happens on my son's computer (desktop, home-built). There's no adverse effect on the performance of the computer, but it's a mystery why it happens and what the computer is actually doing.