DIAGNOSIS That is coil whine from the monitor's internal power supply or driver board. It's a physical vibration in electronic components, often triggered by high refresh rates or specific brightness/contrast settings.
FIX Go into the On-Screen Display menu. Navigate to Display > Refresh Rate and change it from 60Hz to 59.94Hz. This small change often eliminates the resonant frequency causing the whine. If it's set to 59.94Hz, try 60Hz.
IF THAT DIDN'T WORK Adjust the monitor's brightness. Coil whine can be worse at specific brightness levels, often around 50-70%. Try setting brightness to either 30% or 100% to see if it stops.
HEADS UP This noise is generally not a sign of impending failure, just an annoyance. If it's loud and persistent under all settings, contact Dell support. The power supply unit may be defective.