Diagnosis
This is coil whine or capacitor buzz from the monitor's internal power supply or backlight inverter. The load on the electrical components changes with the brightness of the displayed content, altering the audible frequency.
How to Fix It
First, lower the monitor's brightness from the OSD. Open Display > Brightness/Contrast. Reduce Brightness from the default 75% to 60% or 50%. This reduces electrical load on the backlight circuit, often eliminating or significantly reducing the noise.
If That Didn't Work
The issue may be exacerbated by a dirty or unstable AC power source. Plug the monitor directly into a wall outlet, not a power strip or UPS. Test in a different room on a different circuit if possible.
Heads Up
A constant, loud buzzing or a crackling sound is different from variable coil whine. A loud buzz, especially from the rear vent, can indicate a failing component. Contact Dell support if the noise is loud and persistent at any brightness.