makes a high pitched whining noise even at low speed, sounds like it's coming from the back by the battery.

Diagnosis

The high-pitched whining noise emanating from the rear of the DeWalt DCS354B is almost certainly "coil whine" or electronic resonance originating from the Pulse Width Modulation (PWM) circuitry on the motor control board. This occurs when the MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) switch the current on and off thousands of times per second to regulate the tool's speed. This rapid switching creates a magnetic field that causes electronic components—specifically the inductors and capacitors—to vibrate at a frequency audible to the human ear. Because this noise is most prominent at low speeds, it confirms that the PWM controller is operating at a lower duty cycle, which often shifts the frequency into a range that is more perceptible. To confirm this is electronic and not mechanical, run the tool without a blade attached. If the whine persists and remains consistent regardless of the load, the issue is electrical. If the noise changes pitch significantly as you apply pressure to the tool's head, you may be dealing with a bearing failure in the motor housing, though the "back of the tool" location strongly suggests the control board.

How to Fix It

Since this is an electronic phenomenon, you cannot "tighten" a screw to fix it, but you can stabilize the power delivery to minimize the resonance. Follow these steps to isolate and potentially mitigate the noise:

1. Remove the battery and inspect the terminals for any carbon buildup or debris. Use a can of compressed air to blow out the battery port to ensure a clean, low-resistance connection.
2. Swap the current battery for a high-capacity DeWalt 20V Max XR (Extended Runtime) pack, preferably 5.0Ah or higher. Batteries with higher discharge ratings and lower internal resistance provide a more stable voltage curve, which can reduce the "stress" on the PWM controller and lower the audible whine.
3. Perform a "load test" by applying a medium-density sanding disc to the tool. Engage the tool at a low speed and gradually increase to full throttle. If the noise disappears at high speeds, the tool is operating within normal electronic parameters.
4. Check the battery seating. Ensure the battery is snapped firmly into the rails; a loose connection can cause voltage fluctuations that increase the audible frequency of the controller.

If That Didn't Work

• Inspect for Heat Soak: If the whine has suddenly become louder than it was when the tool was new, the capacitors on the control board may be degrading due to overheating. Allow the tool to cool completely for 30 minutes and restart. If the noise is quieter when cold and louder when hot, the board is failing.
• Check for Electrical Interference: Test the tool in a different environment. If you are using the tool near high-voltage equipment or large magnets, the electromagnetic interference (EMI) can exacerbate the resonance in the tool's internal circuitry.
• Internal Debris Removal: Use a high-pressure air compressor to blow through the rear vents near the motor. While the noise is electronic, a buildup of fine drywall or sawdust inside the housing can act as an acoustic amplifier, echoing the electronic whine through the plastic casing.
• Component Replacement: Because the DCS354B uses a sealed electronic module, there are no adjustable potentiometers to change the PWM frequency. If the noise is disruptive and the tool is under warranty, the entire motor and control assembly must be replaced as a single unit.

Heads Up

• Scent Detection: If the high-pitched whine is accompanied by a "sweet" or acrid ozone smell, this indicates a leaking capacitor or a scorched MOSFET. Stop using the tool immediately to avoid a total electrical short.
• Voltage Warning: Never attempt to open the rear housing to "clean" the circuit board with solvents. The capacitors can hold a charge even with the battery removed, and any residue left behind can cause a short circuit upon power-up.
• Battery Health: Avoid using older, degraded batteries with the DCS354B. Low-voltage cells force the controller to work harder to maintain torque, which increases the electronic noise and accelerates wear on the control board.
• Duty Cycle: To prevent the electronics from overheating and increasing the whine, avoid running the tool at the absolute minimum trigger pull for extended periods; this is where the PWM controller is under the most stress.