When I first got a cell phone (an early Sidekick), a new noise entered my life. When I put on my big crazy headphones while the phone was in my shirt pocket, I'd hear a very distinctive "blip ba da blip ba da blip ba da buzzzzzzz" noise right before my phone rang, so loud that I'd have to wrench the headphones from my poor ears -- which was handy, because then I'd hear the phone ringing. It took me a long time to figure out that this noise was coming from the phone, because I'm kind of slow. I also heard it when the phone was near mostly any speakers (even my TV set), including when I received text messages or used other data features on the phone. I recently saw a discussion of these noises on a fellow _flosser's blog post. So what causes this noise?

Basically, it's the cell phone talking to the tower, and nearby speakers picking up that radio transmission. There are several Metafilter articles on the subject, which make for good background reading. The best technical explanation I've seen of the phenonemon I've seen is on this WiFi-Forum post:

The type of interference can occur if the following things happen
1) a pulsing radio transmitter,
2) with relatively strong power,
3) in very close proximity,
4) to a non-linear circuit element.

The non-linear circuit element is usually some sort of solid state
device such as a transistor or diode. If the non-linear element is
subjected to a strong pulsing radio signal, it will act as a rectifier
and "detect" the pulsating waveform, i.e., convert the pulsations from
a radio frequency to an audio frequency (if the pulsation rate is in
the pass-band of audio frequencies.) For example, a hearing aid
consists of a microphone, an audio amplifier and a small speaker. If
a strong pulsating radio signal impinges upon the first transistor
amplifier stage, the transistor will be driven into its non-linear
range and detect the pulsations. If the pulsation rate is in the
audio frequency range, the rest of the hearing aid amplifier will
amplify this and deliver it to the speaker, to the great annoyance of
the hearing aid wearer.

This annoyance is endemic to certain digital cellular technologies (including ones used in music devices like the iPhone, eek). The only ways I've found to mitigate the sound are: move the phone and the speakers father apart (this only seems to reduce the noise a bit...), turn off the cellular portion of the phone (the iPhone, for example, has an "airplane mode" that makes it practical to play music in the car -- without this turned on, the car sounds like it's being ripped apart by buzz-saws), or introduce electromagnetic shielding (good luck building your Faraday cage).

If you've got tips on how to reduce this noise, or a story of how annoying it is, please share in the comments.