DIAGNOSIS That is coil whine from the power supply components. It's common with internal power supplies and is often related to the brightness setting.
FIX Adjust the screen brightness. Press the joystick. Go to Menu > Brightness/Contrast. Change the brightness level up or down. Often, setting brightness to 75% or 25% reduces or eliminates the whine.
IF THAT DIDN'T WORK The whine can be worse with certain refresh rates. Go to Menu > Display > Refresh Rate. Try switching between 60Hz and 59.94Hz if available. Also, ensure Adaptive-Sync is turned off in the Gaming menu.
HEADS UP Coil whine is not a sign of impending failure, but it can be annoying. If the noise is loud and changes with on-screen movement, contact Dell support. It may indicate a faulty power board.