All kidding aside...
Are the CB's loose? (Like the old GE, they had problems) Even with a tight connection on the buss, older GE type breakers were loose enough to buzz under load... Got a bad rep from 'em, but have improved their product dramatically, in my opinion.
Is the vibration at 60 hertz?
The reason I ask, the Xformer for the music festval had a buzz at 120 hertz without any load (for those that don't know me, I'm a musician, trust me) which would be the first harmonic (wouldn't it?) I found that odd, and there was some hum on the sound system too, but at 60 hertz. (all sound was on one buss)
Most 60 cycle related vibration I've run into is usually at the 60 hertz fundamental frequency, the usual fluoros, dimmers, Xformers, etc.
I've heard breakers buzz under overcurrent situations right before they trip. Always when exceeding the rating of the OCPD, not under normal operation other than the GE's mentioned before, and they were giving 80%+ of their rated ampacity.
I think Bill has the right idea about it being similar to the whipping effect the filament in a bulb may experience.
Am I close?
