If you hear a high-pitched sound when flying a brushed micro quad, this could result from the frequency of the motor control signal, the so-called PWM (pulse-width modulation) rate. If this rate is set in the audible spectrum, some people can hear it and find it annoying. If your flight controller is programmable you can set the PWM rate to 16.000 or above, the noise will not beaudible to most people, but you might loose a little bit of peak motor performance.
For more info on this topic, please follow this forum thread! Read on..