DIAGNOSIS The noise is from the motor's armature spinning. The most likely cause is dried-out or failing motor bearings, or lack of lubrication on the armature shaft.
FIX Remove the battery. Check the motor fan at the rear of the tool for obstruction. Spin it by hand. If it grinds or is noisy, the rear bearing is shot. The motor assembly must be replaced as a unit.
IF THAT DIDN'T WORK The planetary gear set in the front gear housing could be dry. A high-pitched whine under load can be metal-on-metal gears. This requires disassembly and specific grease (Bosch lubricant 2 607 001 005).
HEADS UP Do not spray lubricant like WD-40 into the motor vents. It attracts dust and will destroy the motor brushes and commutator. Use only designated electric motor grease.