Diagnosis
This is coil whine. Electrical components on the monitor's internal power board vibrate under load. Changing the on-screen content changes the load.
How to Fix It
First, ensure the monitor's power cable is plugged directly into a wall outlet, not a power strip or surge protector with noisy filtering. Sometimes clean power reduces the buzz. If that fails, the only fix is to mask it. Set a high, constant brightness level (like 80%) for all use. The whine is often worst at mid-to-low brightness.
If That Didn't Work
If the buzz is loud and high-pitched, it's a manufacturing defect. Contact Acer support while under warranty. They may classify it as abnormal acoustic noise and authorize a repair or replacement.
Heads Up
Do not attempt to open the monitor case to fix coil whine. You risk electric shock from the internal power supply capacitors, even when unplugged.