weird high pitched whining noise from my bosch drill when i pull the trigger, even without load.

Diagnosis

The high-pitched whining noise in the Bosch PS31-2A is typically caused by a misalignment or failure in the motor's rotational stability, specifically within the armature bearings or the planetary gear assembly. Because this noise occurs even without a load, it indicates a mechanical friction issue that scales with RPM. When the armature spins at high speeds, any slight eccentricity or lack of lubrication in the bushings creates a high-frequency vibration that manifests as a "whine" or "scream" rather than a grind. This is often caused by "bearing burnout," where the factory grease has either dried out due to heat or been contaminated by fine drywall dust and metallic shavings. If the noise changes pitch significantly when you toggle the speed switch or shift from the drill setting to the screw setting, the issue is likely located in the planetary gear train. If the pitch is constant regardless of the gear selection, the root cause is almost certainly the motor's internal bushings or a failing commutator brush creating electrical interference (EMI) that sounds like a mechanical whine.

How to Fix It

Since the PS31-2A uses a compact, integrated motor housing, you cannot easily replace individual bearings; you must address the assembly as a whole. Follow these steps to isolate and resolve the noise:

1. Remove the 12V battery pack to ensure the trigger cannot be accidentally engaged.
2. Using a T10 Torx driver, remove the outer casing screws. Carefully pry the clamshell housing apart, taking note of the trigger switch wiring to avoid pinching the leads.
3. Locate the motor assembly at the rear of the tool. Gently rotate the armature shaft by hand. If you feel any "catch" or "graininess," the bearing is compromised.
4. Apply a small amount of high-temperature synthetic lithium grease to the armature shaft where it meets the rear housing. Do not over-apply, as excess grease can migrate into the commutator and cause sparking.
5. Inspect the planetary gear set at the front of the tool. Open the gear housing and check for missing teeth or metal shards. If the gears are dry, apply a thin layer of molybdenum disulfide grease to the planetary carrier.
6. Reassemble the housing, ensuring the trigger switch is seated properly in its molded slot before tightening the T10 screws.

If That Didn't Work

• Replace the Motor Assembly: If the whine persists after lubrication, the internal bushings have likely worn oval. You will need to replace the entire motor unit (Part No. Bosch 12V Max Motor Kit). This involves disconnecting the two lead wires from the trigger switch and swapping the motor housing.
• Check for Gearbox Misalignment: If the noise only occurs in high-speed mode, the planetary gear set may be slipping. Check the gear selector switch for debris; if a small pebble or metal shard is lodged in the selector, it can prevent the gear from fully engaging, causing a high-pitched "slip" whine.
• Inspect the Chuck Collet: Occasionally, the whine is not the motor but the chuck collet vibrating at high RPM. Remove the chuck and spin the spindle. If the noise disappears, the chuck is unbalanced and needs replacement.
• Verify Battery Voltage Stability: A failing battery cell can cause the motor to "hunt" or oscillate rapidly, creating a high-pitched electronic whine. Test the tool with a known-good, fully charged 12V Max battery to rule out electrical ripple.

Heads Up

• Avoid using WD-40 or thin penetrating oils on the gears; these will dissolve the heavy-duty grease and lead to total gear failure within a few hours of use.
• When cleaning the motor, use compressed air to blow out the commutator slots. Carbon dust buildup can create a "whining" electrical arc that mimics mechanical noise.
• Ensure the casing screws are tightened evenly. Over-tightening one side of the PS31-2A housing can slightly warp the frame, putting lateral pressure on the motor shaft and causing a whine.
• Keep the vents at the rear of the tool clear. The 12V Max series relies on a small internal fan; if the vents are clogged, the motor runs hot, which accelerates bearing grease degradation.