DIAGNOSIS: A foreign object (like a broken brush head stem or hardened toothpaste) is caught in the mechanism, or the internal motor bearings are failing.
FIX: Immediately remove the brush head. Inspect the metal drive shaft inside the handle for any debris. Use a toothpick or compressed air to carefully clear anything stuck in the socket. Operate the brush without a head attached. If the noise is gone, the problem was debris.
IF THAT DIDN'T WORK: If the loud grinding persists with no head attached, the motor assembly is damaged. This is not repairable. Stop using it, as it can completely seize up or overheat.
HEADS UP: This is often caused by forcing a brush head onto the shaft when it's misaligned. Always align the head with the flat side of the shaft before pushing.