Diagnosis
You have a failing pinion bearing or output shaft bearing. Grinding dust gets past the seals and destroys the bearing balls and races.
How to Fix It
The gear head must be disassembled. Remove the wheel guard and spindle lock nut. Remove the four long bolts holding the gear head to the motor housing. Press out the old bearings. Install new sealed bearings (part 6900 for the pinion shaft, part 6001 for the output shaft). Pack the gear case with fresh grease.
If That Didn't Work
The noise could be from worn helical gears. Inspect the teeth for chipping or excessive wear. Replace the gear set if damaged.
Heads Up
Do not run it with failing bearings. The heat can warp the gear case, turning a bearing job into a full housing replacement.