Found this information on "humming" in electrical equipment at the Square D website.
"...Square D has investigated several instances of reported excessive noise/humming. In most of these instances, loose hardware, wiring, and current transformers have been found to be the cause. In the few instances that this was not the case, the circuit breakers were evaluated and found to have acceptable performance and service life.
No evidence has been found to indicate that operational noise/humming is detrimental to the performance or service life of the circuit breaker..."
http://www.squared.com/us/products/circuitb.nsf/DocumentsByCategory/24B5D9B04636541885256D030061C8A6
