the trimmer makes a really loud grinding noise from the gearbox area, is it broken

Diagnosis

The loud grinding noise emanating from the gearbox of the Milwaukee M18 FUEL String Trimmer (2725-20) is typically caused by a failure in the helical gear set or the loss of high-viscosity lubricant within the sealed gear housing. Because this tool uses a high-torque brushless motor, any misalignment or wear in the gear teeth results in a metallic, high-pitched grinding sound that increases in intensity as the RPMs rise. This is often the result of "gear pitting" or a chipped tooth on the drive gear, which creates a rhythmic grinding sensation felt through the shaft. To confirm the source, disconnect the M18 battery and rotate the trimmer head by hand. If you feel a "catch" or a gritty sensation while rotating, the internal gears are physically damaged. If the rotation is smooth but the grinding only occurs under power, the lubricant has likely leaked out or degraded, causing metal-on-metal friction. Check the area around the gear head for any signs of grease leaking from the seals; a visible oily residue on the outer casing is a definitive sign of seal failure leading to gear starvation.

How to Fix It

Since the 2725-20 gearbox is a factory-sealed unit designed for maximum torque transfer, it is not designed for field-stripping. However, if the unit is out of warranty and you are attempting a recovery, follow these steps to inspect and address the lubrication:

1. Remove the M18 battery and the trimmer head assembly to isolate the gear case.
2. Carefully remove the four Torx screws securing the gear housing to the main shaft.
3. Slide the gear housing off the shaft. Inspect the helical gears for missing teeth, silver scoring, or metal shavings in the remaining grease.
4. If the gears are intact but dry, clean out any contaminated grease using a lint-free cloth and a mild degreaser.
5. Apply a high-pressure, lithium-based gear grease (specifically a tacky, heavy-duty lubricant) to the gear teeth, ensuring full coverage of the helical grooves.
6. Reassemble the housing, ensuring the seals are seated flush against the shaft to prevent future lubricant leakage.
7. Tighten the Torx screws in a cross-pattern to ensure even pressure on the housing seals.

If That Didn't Work

• Replace the Gear Head Assembly: If you find metal shards or chipped teeth during the diagnosis, lubrication will not fix the problem. You must replace the entire gear head assembly. Attempting to replace individual gears in this model often leads to alignment issues that can burn out the brushless motor.
• Inspect the Drive Shaft: Check the drive shaft for any slight bends or warping. A bent shaft causes the gears to mesh at an angle, creating a grinding noise even if the gears themselves are healthy. If the shaft is bent, the entire upper arm assembly must be replaced.
• Check for External Debris: Ensure that a piece of heavy-duty plastic or a thick weed hasn't wrapped itself around the gear head shaft behind the head assembly. This can create a high-frequency vibration that sounds like internal grinding but is actually external friction against the housing.
• Verify Battery Connection: In rare cases, a failing battery cell can cause the motor to "stutter," which sounds like mechanical grinding. Test the tool with a different, fully charged M18 High Output battery to rule out electrical oscillation.

Heads Up

• Avoid using the trimmer in extremely sandy or gritty environments without cleaning the head, as abrasive particles can migrate past the seals and destroy the helical gears rapidly.
• Never use WD-40 or thin oils to lubricate the gearbox; these will wash away the factory grease and accelerate gear wear.
• If the tool is under its 5-year limited warranty, do not open the gear case, as breaking the factory seals will void your coverage for a free replacement.
• Periodically check the trimmer head for excessive play; if the head is loose, it puts lateral stress on the gearbox, which is the leading cause of premature gear failure.