DIAGNOSIS Worn motor bearings or a damaged drive coupler. The coupler's teeth are likely grinding against the blade assembly socket.
FIX Follow the steps for the seized coupler. Replace the drive coupler first. While the coupler is out, spin the motor shaft by hand. It should turn smoothly with minimal noise. Any gritty grinding indicates bearing wear.
IF THAT DIDN'T WORK If noise persists with a new coupler, the bearings in the motor housing are failing. Motor disassembly and bearing replacement is required, which is complex for this model.
HEADS UP Loud grinding noise means stop using it immediately. Running it will cause further, irreparable damage to the motor and blade assembly.