Ron Natalie writes:

You would think that the digital cell phones would be low power
enough, yet my GSM cell phones have always annoyed the hell out
of come low level audio (portable MP3 player docks and the like).
The sound is pretty distinctive. It's even gotten into my airplane
audio. It's a reminder for me to shut off my phone.

I think that interference comes from chip-modulation RFI in the phone.
I note that it's extremely sensitive to distance, which implies that
it has nothing to do with the actual transmitted energy. Laptops and
other types of computers also generate this kind of audio-frequency
interference. I don't think it bothers most avionics, but I'm not
sure (which means that I'd avoid it).

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