Diagnosis
Normal sound for the brushless motor controller. It's the pulse-width modulation at low speeds.
How to Fix It
This is not a defect. No fix is needed or recommended. The electronic controller varies power by rapidly pulsing the motor. This creates the audible frequency.
If That Didn't Work
If the sound changes to a grinding or rattling, that is not normal. Stop use. That indicates a failed motor bearing.
Heads Up
Do not attempt to 'lubricate' the brushless motor. It is sealed. Introducing lubricant will destroy it.