DIAGNOSIS This is coil whine from the monitor's internal power supply or voltage regulation circuits. It is often related to the monitor's brightness/backlight level or the connected video signal's refresh rate.
FIX Adjust the monitor's brightness setting. The whine is often most pronounced at specific brightness levels, typically between 40% and 70%. Incrementally adjust the brightness up or down from your current setting until the noise diminishes or stops. A setting of 30% or 80% often minimizes it.
IF THAT DIDN'T WORK Change the refresh rate. In your operating system's display settings, try switching between 60Hz, 75Hz, and the native 75Hz setting. The electrical load difference can change the whine frequency or eliminate it. Also, ensure Adaptive-Sync/FreeSync is turned OFF in the monitor's OSD under GamePlus > Adaptive-Sync, as this dynamically alters circuitry.
HEADS UP This noise is an inherent characteristic of some electronic components and is not typically a sign of impending failure. It is often more audible in very quiet environments. If the noise is loud or changes to a crackling sound, contact ASUS support, as that may indicate a faulty component.