You are assuming it is a "Buzz" because that's what your ears "Hear" after it comes out of a speaker and after being processed by radio receiving equipment of a certain design.

Perhaps the "Buzz" is not meant to be a sound anyone hears. Maybe it is something more like a dialup modem tone that is acted upon by something other than a shortwave receiver. Maybe there is more to this noise than just buzzing. Maybe the buzzing is a spurious side effect of some other process going on.

Old dialup in audio and visual waveform: http://www.freesound.org/samplesViewSingle.php?id=16475

Do you guys have any thoughts on this?