my 2821-20 makes a really loud grinding or rattling noise when i'm not even cutting, just holding the trigger

Diagnosis

The loud grinding or rattling noise experienced when the trigger is pulled on the Milwaukee M18 Fuel Reciprocating Saw (2821-20) is almost always a symptom of mechanical failure within the drive train's reciprocating assembly. Because the noise occurs without a load (not cutting), the issue is not related to the blade or the shoe, but rather the internal components that convert the motor's rotational energy into linear motion. Specifically, the needle bearings in the counterweight or the main drive bearing on the crankshaft have likely disintegrated or lost their lubrication. When these bearings fail, the counterweight—which is designed to offset the vibration of the blade—begins to oscillate irregularly or "slap" against the internal housing. This creates a metallic rattling sound that increases in frequency as you pull the trigger further. If the noise is more of a high-pitched grind, the drive bearing is likely pitted or seized, causing the shaft to rotate off-center. You can confirm this by removing the blade and the shoe; if the rattling persists and you can feel a distinct vibration in the handle that doesn't match the normal rhythmic pulse of the tool, the internal bearings are compromised.

How to Fix It

To resolve this, you must perform a full teardown of the gear housing to replace the failed bearings. Ensure you have a set of Torx drivers (T15 and T20) and a needle-nose plier.

1. Remove the battery and the blade. Unscrew the shoe bolts to clear the front of the tool.
2. Remove the housing screws on the side of the tool. Carefully peel back the plastic casing, taking care not to pinch the trigger wires or the LED light wiring.
3. Locate the drive assembly. You will see the crankshaft and the counterweight. Use a press or a punch tool to remove the retaining clips holding the counterweight in place.
4. Slide the counterweight off the shaft. Inspect the needle bearing. If you see metal shavings or "brassing" (where the bearing has worn a groove into the shaft), the bearing must be replaced.
5. Remove the main drive bearing from the crankshaft. Check for any axial play; if the shaft wiggles more than 0.5mm in any direction, the bearing is shot.
6. Clean the shaft thoroughly with a lint-free cloth and solvent to remove all metal debris.
7. Install new high-temp needle bearings and a replacement drive bearing. Ensure the bearings are seated flush against the shoulders of the shaft.
8. Re-grease the assembly using a high-pressure lithium-based grease. Do not overfill, as excess grease can migrate into the motor windings.
9. Reassemble the housing in reverse order, ensuring the trigger mechanism is seated properly before tightening the casing screws.

If That Didn't Work

• Check the planetary gear set. If the noise is a consistent "whine" rather than a rattle, one of the small planetary gears may have a chipped tooth. If you find metal shards in the gear oil, the entire gear head assembly needs to be replaced as a single unit.
• Inspect the motor armature for centrifugal imbalance. If the tool was dropped while running, the rotor may be slightly bent. This causes a rhythmic thumping or rattling that sounds like a bearing failure but is actually the rotor hitting the stator. This requires a full motor replacement.
• Examine the trigger switch contacts. In rare cases, a failing switch can cause "chatter," where the motor rapidly cycles on and off. This creates a rapid-fire rattling sound. Test this by checking if the noise persists when using a different battery pack.
• Verify the integrity of the blade clamp mechanism. If the internal spring or the locking dog is worn, the blade holder can vibrate violently against the internal guide, mimicking a gear noise. Replace the blade clamp assembly if there is excessive side-to-side play.

Heads Up

• Avoid "dry-running" the tool for extended periods without a blade; while it doesn't cause immediate failure, the lack of resistance can sometimes exacerbate vibration in a weakening bearing.
• Never use WD-40 as a lubricant for the internal gears; it is a solvent and will strip the factory grease, leading to rapid bearing failure.
• Periodically blow out the motor vents with compressed air. Dust buildup in the motor can cause overheating, which thins the grease in the drive bearings and accelerates wear.
• When replacing bearings, always use shielded bearings to prevent construction debris from entering the race.