Diagnosis
That's likely coil whine from the monitor's internal power supply or components. It's not "normal" in the sense of being intended, but it's a common electronic phenomenon and usually isn't a sign of impending failure.
How to Fix It
First, identify when it happens. Is it constant, or only when the screen is very bright (like a white webpage) or when it's at a specific brightness level? Try adjusting the monitor's brightness via the OSD (not your computer's settings) up or down a few notches. Sometimes changing the refresh rate (in Windows display settings) from 75Hz to 60Hz or vice versa can shift the electrical load enough to stop the noise.
If That Didn't Work
Check your power source. Try plugging the monitor into a different wall outlet or a surge protector, away from other high-draw devices. A dirty or unstable power supply can exacerbate coil whine.
Heads Up
If the buzzing is loud enough to hear over ambient room noise from a normal sitting distance, and it's new, it could be a defect. Contact LG support while under warranty—they may consider excessive audible coil whine a valid reason for a repair or replacement.