DIAGNOSIS That's coil whine, and it's a common electronic phenomenon, not necessarily a defect. It's often caused by the power supply components vibrating at a high frequency, which can change based on the monitor's brightness or the content displayed.
FIX First, try to isolate the cause. Turn the brightness down to 50% or below using the OSD joystick. Does the noise change or go away? Display a completely white screen (like a blank document), then a completely black screen. The pitch often changes with the power load. If adjusting the brightness minimizes it to a tolerable level, that's your simplest fix. Also, ensure the power cable is firmly plugged into both the monitor and a grounded wall outlet, not a power strip, as unstable voltage can exacerbate whine.
IF THAT DIDN'T WORK The whine might be interacting with your environment. Try moving any power bricks, wireless routers, or other electronics further away from the monitor. Sometimes electromagnetic interference from other devices can make the noise more pronounced.
HEADS UP Coil whine is generally harmless and doesn't affect performance or longevity, but it can be a deal-breaker for a quiet workspace. If it's loud and persistent right out of the box, contact ASUS support while you're still within the return or warranty period to discuss options.