As far as I know (which is from a minor amount of personal experience and a major amount from my electrical engineering degree), most humming and crackling soun

As far as I know (which is from a minor amount of personal experience and a major amount from my electrical engineering degree), most humming and crackling sounds are produced by corona discharges and they are generally heard only near overhead power cables operating in high voltage ranges (i.e. 6kV and above). That said, I have heard humming sounds from older tube lights too. Apparently this indicates a faulty choke.