Diagnosis
That's coil whine from the monitor's power supply or internal components. It's common, not a defect that affects function, and is often triggered by specific brightness levels or high refresh rates.
How to Fix It
Adjust the brightness. The whine is often worst at 50% or 100% brightness. Set it to 30% or 80% and see if it stops. Changing the refresh rate from 144Hz to 120Hz in the monitor OSD can also eliminate it.
If That Didn't Work
Ensure the monitor is plugged directly into a wall outlet, not a power strip shared with other high-draw devices. A 'dirty' power signal can exacerbate the noise.
Heads Up
Coil whine can be more noticeable in a very quiet room. If it's loud and persistent at all settings, contact Dell support within the warranty period—they may classify it as a fault and replace it.