From what i´ve read it could be DC on the AC line. Here´s a thought: Maybe there has always been DC on your AC line but your step-up transformer might have been filtered it away (until now). Now think if a capasitor has shorted. Now the DC doesn´t get filtered anymore and the transformer starts to hum. Just an idea...
Here´s a link to someone selling hum eliminators with some info on the isue:
http://www.psaudio.com/articles/hum.asp