I'm not sure what in the head would be humming if you've excluded the fan - it depends what other components are inside as to what it is and what is the cause. Is the hum coming from the power supply or the components within the head? My Durst power supply does hum quite significantly when it's providing power to the lamp but it's normal. If some components in the head are expecting a DC voltage and are receiving some AC then you might have a faulty rectifier or capacitors in the power supply. Does your voltage stabiliser completely replace the supplied transformer? If you're blowing the proper type and rating of fuses in the stabiliser, then as Peter suggests it probably doesn't have the power rating to drive the enlarger. Sorry I can't be more help.

Roger.