The sound that you hear when you cell phone is too close to an audio device.
I recently came across a short post explaining what it is:
“The reason for the buzz is the nature of time-division mulitple access (TDMA). In the US, we operate mobile phones at 850 Mhz and 1900 Mhz; in Europe, 900 Mhz and 1800 Mhz. Good so far; that’s not going to make noise that we can hear. TDMA fits more subscribers into the same bandwidth by assigning different terminals different timeslots (vs. CDMA, which uses black magic). These timeslots happen to be spaced 4.615 ms apart, yielding a signal envelope which looks a lot like a dirty 217 Hz square wave. All sorts of things (like “wires”) are good at picking up a 217 Hz square wave at 0.5 W, and 217 Hz is conveniently smack dab in the middle of our auditory capabilities.”
- Jered
via www.bunniestudios.com/blog/wp-trackback.php?p=210