Electromagnetically Induced Acoustic Noise
en.wikipedia.org
en.wikipedia.org
GSM phones would transmit TDMA frames at a frequency of 217 Hz which couples with audio circuitry to cause speakers to emit a characteristic buzzing sound.
Even though it's a radio frequency signal which is outside the range of human hearing (like 800-900 MHz or higher), the signal interacts with any non-linearity in the audio circuitry (think of the diode in a crystal radio), which basically performs unintended amplitude modulation of the pulsed RF electrical signal into something like a 217 Hz square wave, which is in the audible range.
This has gone away as phones stopped using GSM. Newer standards emit more of a continuous waveform which doesn't cause these audible frequencies to be generated.
It's really ,really distracting and enervating. Specially, because most people around you can't ear it and think you are just being picky. Imagine a TV, and that noise is there all the time and you can't watch anything in peace because you keep earning that high pitched noise in the background.
A few years ago, I fixed one with part of a bean can.