A DC offset on the mains can cause hum. Usually caused by a local industry pulling power out on half cycles, so either the top or the bottom of the mains waveform is flattened (but not both, or not equally). What happens is that the effective DC offset biasses the core of the transformer, and you hear harmonic distortion through magnetostriction in the core.
There is a good article, and method to remove it here http://sound.westhost.com/articles/xfmr-dc.htm
Basically two big electrolytics in opposed series, or back to back parallel, with diode clamps across them. In series with the live power. Worth a bash since some have silent transformers and others don't, which could well come down to local variation of mains quality.