Diagnosis
A loud grinding noise originates from the gear case. This indicates a lack of lubrication or mechanical failure within the helical gear assembly.
How to Fix It
Stop using the tool immediately. The gear case is sealed and lubricated at the factory. No user-serviceable parts exist inside. This condition requires warranty or repair service. Contact Milwaukee Tool.
If That Didn't Work
Rule out a simple cause first. Ensure the noise is not coming from the trimmer head itself scraping against a hard surface or a loose debris shield. Remove the spool and run the tool briefly. If the grinding persists, it is the gearbox.
Heads Up
Continuing to use the tool with a grinding gearbox will cause catastrophic failure. It will shear the gears, requiring a complete gear case replacement, which is more costly than an early warranty claim.