Diagnosis
Mechanical interference in the drive train. The brush head is not properly seated or is damaged.
How to Fix It
Remove the brush head. Inspect the plastic socket for cracks. Inspect the metal drive pin on the handle for bending. Firmly reattach a known-good, official brush head until it clicks into place. Test.
If That Didn't Work
Foreign material inside the brush head socket. Use compressed air or a toothpick to clear any debris from the small hole where the drive pin inserts. Reattach the head.
Heads Up
A persistent grinding noise after trying a new head indicates internal motor gear damage. The handle is not user-serviceable and requires replacement under warranty or via the refill plan.