DIAGNOSIS This is normal operation for the brushless motor and electronic controller. The whine is the frequency of the electronic switching (PWM) that controls motor speed. It is most audible at low RPMs.
FIX No fix is required or possible. The noise is a characteristic of the brushless motor design. It does not indicate wear or impending failure. You can confirm by running the drill at different trigger pressures in speed setting '1'; the pitch of the whine will change with speed.
IF THAT DIDN'T WORK If the whine is accompanied by a grinding, scraping, or clicking sound, that is a mechanical issue. Isolate the sound: run the drill without a bit. A pure electronic whine is normal. Any other noise is not.
HEADS UP The noise may be more pronounced when the battery charge is low. The electronics work harder to maintain performance, which can alter the audible frequency. Recharge the battery to see if the sound character changes.