What is this noise?

Every time I'll be watching a KC-135 video and hear the aircrew talking or the boom operator talking to the trailing plane, there is always this background hum or buzzing in the background of the radio comm. I hear this in pretty much all videos with radio communication for air crew. Just out of curiosity, does anyone actually know where this comes from or what is causing it? Is there a name to describe this sound? Is it some sort of interference?

I put together a few clips of the sound I'm talking about so you can hear it:

It has to do with the electrical system on these jets... The AC generator's output runs about 400hz, which comes across the radio when they speak.

I think the newer jets seem to have better filtering that cuts that noise out better on frequency. The ol' 747s and DC-10s really seem to be noticeable more to me. :-)

Every time I'll be watching a KC-135 video and hear the aircrew talking or the boom operator talking to the trailing plane, there is always this background hum or buzzing in the background of the radio comm. I hear this in pretty much all videos with radio communication for air crew. Just out of curiosity, does anyone actually know where this comes from or what is causing it? Is there a name to describe this sound? Is it some sort of interference?

 

I put together a few clips of the sound I'm talking about so you can hear it: