my drill makes a high-pitched whining or grinding noise when it's under load, sounds bad.

Diagnosis

The high-pitched whining or grinding noise occurring specifically under load in the Bosch GSR12V-300B22 is typically caused by a failure in the planetary gear reduction system or a misalignment in the brushless motor's bearing assembly. Because this is a brushless model, the electronic commutation is silent; therefore, any audible "screeching" or "grinding" is mechanical. When the drill is under load, the torque increases the pressure between the gear teeth. If the factory lubricant has degraded or if a gear tooth has chipped, the friction increases exponentially, creating that distinct high-pitched whine. Another common culprit is the ingress of fine construction dust (drywall or sawdust) into the gear housing. These particles mix with the grease to create an abrasive paste that wears down the planetary rollers. You can confirm this by switching between the low-torque (Gear 1) and high-speed (Gear 2) settings. If the noise pitch changes drastically or disappears in one specific gear, the issue is isolated to the transmission gears. If the noise persists across all speeds and sounds like a metallic "shriek," the front spindle bearing is likely shot.

How to Fix It

To resolve this, you must inspect the gearbox and replenish the lubrication. If the gears are not physically chipped, a high-viscosity synthetic grease replacement often solves the whining.

1. Remove the battery pack and the bit from the chuck to ensure a safe working environment.
2. Using a T10 Torx driver, remove the housing screws. Carefully split the clamshell casing, taking note of the trigger assembly wiring to avoid pinching it.
3. Locate the planetary gear assembly at the front of the tool. Inspect the gear teeth for "pitting" (small holes in the metal) or sheared edges.
4. If the gears look intact but the grease is grey or blackened, remove the old lubricant using a lint-free cloth and a small amount of isopropyl alcohol.
5. Apply a high-pressure, lithium-based synthetic gear grease (such as Molykote or a similar industrial grade) to the planetary gears. Ensure the grease is packed into the teeth and doesn't overflow into the motor housing.
6. Reassemble the casing, ensuring the gear selector switch is properly seated in its notch before tightening the T10 screws.
7. Run the drill in Gear 1 for 30 seconds, then Gear 2, to distribute the grease before applying a heavy load.

If That Didn't Work

• Replace the Planetary Gear Set: If you find physical chips or "flat spots" on the planetary rollers, lubrication will not fix the noise. You will need to source a replacement gear carrier assembly. Ensure the new gears are timed correctly with the sun gear to avoid jamming the motor.
• Spindle Bearing Replacement: If the noise is a high-pitched whistle regardless of the gear setting, the front bearing in the chuck spindle has failed. Press out the old bearing using a small arbor press or a punch and hammer, and install a new precision ball bearing. Ensure the bearing is seated fully to avoid chuck wobble.
• Motor Bearing Lubrication: In rare cases, the rear bearing of the brushless motor loses lubrication. Apply a single drop of high-speed precision oil (like sewing machine oil) to the rear motor bearing. Do not use heavy grease here, as it will impede the high-RPM rotation of the brushless armature.
• Check for Chuck Obstruction: Remove the chuck entirely and run the drill. If the noise disappears, the internal mechanism of the keyless chuck is worn or clogged with debris, causing it to vibrate against the spindle under load.

Heads Up

• Avoid using WD-40 or thin penetrating oils inside the gearbox; these will strip the existing lubricant and accelerate gear wear, leading to total transmission failure.
• Always clear the vents of the brushless motor with compressed air every 20-30 hours of use to prevent overheating, which can thin the gear grease and cause it to leak out of the housing.
• If you notice "slop" or play in the chuck while the noise is occurring, stop using the tool immediately to prevent the planetary gears from stripping completely.
• When reassembling the casing, do not over-torque the T10 screws, as the plastic bosses on the GSR12V series are prone to stripping, which will compromise the tool's structural integrity.