DIAGNOSIS: Sawdust and debris have infiltrated the vent slots and are likely coating the motor and gears, causing extra friction and that gritty sound.
FIX: Never submerge it. First, remove the battery. Use compressed air from a compressor or canned air, and blow it into all the vents—especially near the motor housing and the front selector ring—while holding the trigger down to let the motor spin freely. Do this from multiple angles.
IF THAT DIDN'T WORK: For compacted dust, you might need to partially disassemble it. Removing the outer housing (a bunch of Torx screws) will let you brush and vacuum out the bulk of the debris. Be careful of the spring-loaded brushes in the motor.
HEADS UP: Avoid using a vacuum directly on the vents; it can create static electricity. Compressed air is safer. Doing this regularly, especially after dirty jobs, is the best prevention.