my milwaukee hammer drill makes a horrible grinding noise only when i push the hammer mode button, drilling is fine

Diagnosis

The grinding noise occurring exclusively in hammer mode on the Milwaukee M18 FUEL 2904-20 indicates a failure within the percussion assembly, specifically where the drive gear interfaces with the hammer ram and the anvil. In standard drilling mode, the tool operates on a continuous rotational path. However, when you toggle the hammer mode switch, the tool engages a secondary mechanism that converts rotational energy into a reciprocating linear motion. If the high-viscosity grease in this specific chamber has been displaced by centrifugal force or contaminated by fine masonry dust, you will hear a metallic, rhythmic grinding or "chattering" sound.

This is often caused by "dry-running" the hammer mechanism. Over time, the heat generated by the rapid-fire percussion (thousands of blows per minute) thins the factory lubricant, which then migrates away from the strike face. If you have been drilling into abrasive materials like concrete or brick without intermittent breaks, the dust can enter the housing and mix with the remaining grease, creating an abrasive paste that wears down the tolerances of the ram. You can confirm this by running the tool at low RPM in hammer mode; if the noise is a consistent, high-pitched metallic scream, it is likely a lubrication failure. If it is a chunky, irregular clunking, a bearing or a gear tooth in the percussion assembly may be chipped.

How to Fix It

To resolve this, you must perform a deep cleaning and re-greasing of the percussion assembly. You cannot simply spray lubricant into the vents; you must access the internal gear train.

1. Power down the tool and remove the M18 battery pack immediately to prevent accidental trigger pulls.
2. Using a T15 Torx driver, remove the housing screws. Be mindful of the screw lengths, as the screws near the trigger assembly are shorter than those on the rear motor housing.
3. Carefully split the clamshell housing. Gently pull the two halves apart, ensuring you do not pinch or sever the wires leading to the brushless motor or the LED ring.
4. Locate the hammer mechanism—the heavy steel assembly located behind the chuck. Use a microfiber cloth and a small amount of isopropyl alcohol to wipe away the old, contaminated grey/black grease and any accumulated masonry dust.
5. Apply a high-pressure, lithium-based multipurpose grease (such as Mobilgrease XHP or a dedicated gear grease). Apply a generous bead specifically to the hammer ram and the drive gear interface. Do not overfill to the point where grease touches the motor commutator, as this can attract debris and cause overheating.
6. Reassemble the housing, ensuring the trigger switch is seated correctly in its notch. Tighten the T15 screws in a crisscross pattern to ensure an even seal against dust ingress.

If That Didn't Work

• Check for a Worn Hammer Spring: If the grinding is accompanied by a lack of actual percussion (the drill spins but doesn't "hammer"), the internal return spring may have snapped or lost tension. This requires a full replacement of the percussion assembly, as the spring is integrated into the ram housing.
• Inspect the Drive Gear Teeth: Examine the teeth on the gear that engages the hammer mode. If you see "silvering" or metal shavings (swarf) in the grease, the gear teeth have been stripped. You will need to replace the gear set (Part # 45-01-xxxx series depending on the specific build date).
• Chuck Alignment Check: Occasionally, the noise is actually the chuck wobbling under the vibration of hammer mode. Remove the chuck and check the spindle for play. If the spindle has axial play of more than 0.5mm, the internal bearings are shot and the front gear case must be replaced.
• Switch Failure: If the noise is an electrical "buzzing" rather than a mechanical grind, the hammer mode selector switch may be failing to fully engage, causing the gears to "slip" or ride on the edges of the teeth. Replace the mode selector switch assembly.

Heads Up

• Avoid "Dry Hammering": Never run the tool in hammer mode without a bit installed and pressed against a workpiece for more than a few seconds; the lack of resistance can cause the hammer mechanism to vibrate violently, accelerating wear.
• Dust Management: When drilling masonry, use a vacuum or blow out the tool's vents with compressed air every 15 minutes to prevent dust from being sucked into the gear housing.
• Grease Compatibility: Never use WD-40 or thin penetrating oils inside the gear head. These will dissolve the remaining factory grease and lead to total mechanical failure within hours of use.
• Heat Warning: If the gear housing feels excessively hot to the touch after 5 minutes of hammer use, stop immediately. This indicates a mechanical bind or lack of lubrication that will lead to permanent gear deformation.