This one has been driving me mad for years. Clicking on modifier keys on the macOS Accessibility Keyboard would make this awful typewriter clunking sound.

How to make it stop:

• Go to System Settings > Accessibility > Keyboard.
• Click the little info icon next to Accessibility Keyboard
• Make sure Play sounds for key and dwell actions is off (it probably is already)
• Click Done, then click the info icon next to Sticky Keys.
• Turn off Beep when a modifier key is set.

You might be thinking, “But sticky keys isn’t even on!” To which I respond, yes that’s why it took me so long to find that setting and why I’m writing it down on the interwebs.