DIAGNOSIS The main drive gear assembly has worn out or a gear tooth has sheared off. This is a common wear item on this series after high page counts.
FIX The gear assembly (Brother part # LJ4514001) needs replacement. This requires full disassembly to access the right-side gear train. The old gear must be removed and the new one seated precisely on its shaft.
IF THAT DIDN'T WORK Check the fuser drive gear. If the main gear was damaged, it may have also damaged the mating gear on the fuser unit. Inspect the fuser gear for missing teeth.
HEADS UP Stop using the printer immediately when you hear the grind. Forcing it will cause more expensive damage to the motor and other gears.