DIAGNOSIS Yeah, this one's common. It's coil whine from the power supply or backlight circuitry inside the monitor. It's not dangerous, just annoying.
FIX The whine often changes with the brightness setting or the on-screen content. Try lowering the brightness from the OSD menu (the physical buttons on the back). Set it to 80 or below and see if the noise diminishes. Sometimes enabling the "OD" (Overdrive) setting in the OSD under Image > Trace Free can shift the electrical load and change the noise.
IF THAT DIDN'T WORK If the noise is clearly coming from the back and is more of a loud buzz, it could be a faulty internal power board. While you can't fix that yourself, check if the monitor is still under warranty (ASUS typically has a 3-year warranty for panels). Contact ASUS support—persistent, loud buzzing is a valid RMA reason.
HEADS UP Don't try to open the monitor to "fix" the noise. There's a high-voltage capacitor inside that can hold a dangerous charge long after it's unplugged. You'll also void the warranty instantly.