DIAGNOSIS That's coil whine. It's caused by electrical components (like capacitors or inductors) on the monitor's internal power board vibrating at a high frequency. It's not dangerous, but it can be annoying. It often gets louder when the screen is very bright or when the refresh rate is at its peak (75Hz for your model).
FIX First, try changing the refresh rate. On Windows, go to Display Settings > Advanced Display > Choose a refresh rate, and select 60Hz instead of 75Hz. See if the noise diminishes. Next, adjust the monitor's brightness. Go into the OSD menu (the joystick), go to 'Picture', and lower the 'Brightness' and 'Contrast' from their max settings. A less intense image draws less power and can quiet the whine.
IF THAT DIDN'T WORK The whine might be related to FreeSync. Go into the monitor's 'Game' menu and turn 'FreeSync' off. If the noise stops, you've found the culprit. You can leave it off, or you might just have to live with a faint noise when it's enabled.
HEADS UP If the noise is a loud hum or crackle, or if it's accompanied by a burning smell, that's a different, serious issue. Power off and unplug the monitor immediately—that points to a failing power supply that needs repair.