Diagnosis
A foreign object is caught in the brush mechanism, or a plastic gear inside the drive housing is damaged.
How to Fix It
First, check for obstructions. 1) Turn the mop over and remove the scrubbing pad. 2) Look around the rotating brush head for any pebbles, paper clips, or bits of plastic. 3) Use your fingers or tweezers to remove anything you see. 4) Manually spin the brush head with your hand to feel for any rough spots or catches.
If That Didn't Work
The noise may be internal. This often requires removing a few screws to take off the brush head cover to inspect the gears. If you see broken plastic teeth, the gear assembly will need replacement.
Heads Up
Always do a quick visual sweep of the floor before you mop. This machine is great, but it will happily try to eat small, hard objects, and that rarely ends well for the gears.