DIAGNOSIS A high-pitched whine is typically electronic noise from the motor controller, not mechanical. It's often normal but can indicate a failing component.
FIX First, try a different DeWalt 20V MAX battery. A battery with low cells or high internal resistance can cause the electronics to whine. If the noise changes or goes away, the battery is the issue.
IF THAT DIDN'T WORK The noise is likely from the Field Effect Transistors (FETs) on the control board. If it's a new, loud sound, the board may be stressed. There is no user fix.
HEADS UP If the whine is accompanied by a burning smell or reduced power, stop use immediately. This indicates a failing component that could overheat.