there's a high pitched whining noise coming from my drill when i pull the trigger lightly, goes away at full speed.

Diagnosis

The high-pitched whining sound you are hearing in your DeWalt DCD800B is not a mechanical failure, but rather an electronic byproduct of the Brushless DC (BLDC) motor system. Unlike brushed motors that use physical carbon brushes to transfer current, the DCD800B utilizes an electronic controller to switch current to the motor windings. To achieve the variable speed control provided by the trigger, the tool uses a technique called Pulse Width Modulation (PWM). PWM works by rapidly switching the power on and off thousands of times per second. When you pull the trigger lightly, the "off" periods are longer than the "on" periods. This rapid switching creates a high-frequency electromagnetic vibration in the motor windings and the internal circuitry, which manifests as an audible high-pitched whine or "singing" noise. As you pull the trigger further and approach full speed, the duty cycle increases until the power is nearly constant, which is why the noise disappears or blends into the general roar of the motor and gearbox at high RPMs. You can confirm this is the PWM signal if the pitch of the whine changes proportionally with the trigger pressure. If the noise is accompanied by a grinding sound, excessive vibration in the chassis, or a smell of ozone, that would indicate a bearing or gear failure; however, a clean, consistent electronic whine is a standard characteristic of the DCD800's power electronics.

How to Fix It

Because this is a fundamental design characteristic of the brushless controller, there is no "repair" to remove the noise entirely. However, you can optimize the tool's operation to minimize the audible frequency and ensure the controller is performing efficiently:

1. Optimize Gear Selection: Switch the gear selector on top of the drill to Position 1 (Low Speed/High Torque). Because the motor doesn't have to spin as fast to achieve movement in Gear 1, the PWM frequency is often shifted to a range that is less piercing to the human ear compared to Gear 2.
2. Calibrate Trigger Pressure: Avoid "feathering" the trigger at the absolute minimum threshold for extended periods. Quickly transitioning through the low-voltage start-up phase to your desired working speed reduces the time the motor spends in the most audible PWM range.
3. Verify Battery Contact: Ensure your 20V Max battery is fully seated and the terminals are clean. Low voltage or poor contact can sometimes cause the controller to struggle, potentially altering the pitch or intensity of the electronic whine.
4. Load Application: Apply a small amount of pressure to the workpiece before pulling the trigger. When the motor has a slight load, the electronic load on the controller changes, often dampening the resonance of the motor windings.

If That Didn't Work

If the noise is accompanied by performance drops or sounds "mechanical" rather than electronic, try these alternatives:

• Check for Gearbox Contamination: If the whine is accompanied by a rhythmic clicking, a small amount of debris may have entered the planetary gear set. While the DCD800 is sealed, high-pressure air can be used to blow out the vents around the motor housing to clear any metallic dust that might be causing resonance.
• Test with a Different Battery Pack: Switch from a compact 2.0Ah battery to a high-capacity 5.0Ah or 6.0Ah battery. Higher amperage delivery can sometimes stabilize the voltage curve during the initial trigger pull, slightly altering the PWM noise profile.
• Inspect the Chuck for Obstructions: Ensure there is no debris inside the chuck jaws. A slightly off-center bit or a foreign object in the chuck can create a harmonic vibration that amplifies the electronic whine of the motor.
• Cycle the Gear Switch: Move the gear selector rapidly between 1 and 2 several times. If the whine changes drastically or disappears in one specific gear, it may indicate that the gear alignment is slightly off, though this is rare in the DCD800 series.

Heads Up

• Avoid Over-Torquing: While the DCD800 is powerful, consistently pushing the tool to its absolute torque limit in Gear 1 can overheat the controller, which may eventually lead to a more pronounced electronic hum or premature board failure.
• Battery Health: Always use genuine DeWalt 20V Max batteries. Third-party "knock-off" batteries often have inconsistent voltage regulation, which can cause the PWM whine to become erratic or significantly louder.
• Heat Dissipation: If the tool feels excessively hot to the touch near the trigger area, stop use immediately. Overheating the electronic controller can permanently warp the capacitors, leading to erratic speed control and increased noise.