Home Breaker Box Buzzing

Hello all. I have a tough one. I moved to a new home (no previous owner) and detected intermittent buzzing in my computer speakers. The buzz comes and goes, always lasts between 5 and 15 seconds, and tends to group up -- that is, i'll hear maybe 15 buzzes in an hour, then nothing for a couple hours. It occurs at any time of day.

I've probably spent a month or so trying various things to work it out, such as isolating appliances, turning things like the AC off, putting devices on different circuits. I tried to isolate the issue and one day noticed the same buzz was actually audible in my breaker box!

I've had an electrician come out twice, both times tightening the screws for the wiring at the box and the power source outside the home. They suggested it might be one of the breakers, so I methodically switched off each one mid-buzz to see which one, but it isn't any of them.

My residence is a town-home, so I coordinated with my next-door neighbors (great people) and found they also have a buzz, though maybe not quite as loud. My unit is closer to the source power, which might explain it.

Please find the buzz, which i recorded of the breaker box buzzing. The buzzing is at the 0:30 and 1:00 marks, a coincidence i think as it doesn't always happen in 30 second intervals.

https://www.dropbox.com/s/fa33eiqa6l45ea5/breaker-buzz.mp4?dl=0

Any suggestions or help is welcome!