Why does my work monitor buzz when there's a lot of black on the screen?

Whenever I first turn my computer on and my black desktop pops up, my monitor makes this loud buzzing noise. As soon as I open up a webpage or cover up a good portion of the screen with a folder, it stops. If everything’s closed or I open up a webpage that’s mostly black or a dark color, it starts up again.

It’s a fairly new LCD monitor, and with the exception of the buzzing the monitor works great. Any ideas on what could be causing it, and what I could do to fix it?