You can sometimes get transformer-hum from electrical equipment.
Transformer iron cores are always made of thin plates riveted together. Sometimes the rivets don't hold as well as they should and the resulting very tiny movements of the plates produce an audible hum as they rub together.
I have cured this in the past by tapping a transformer quite sharply with a spanner - sometimes it has no effect - sometimes it works - sometimes it make the noise louder.
Your hum may be something entirely different - you may consider using a wooden spoon as a stethoscope to discover the source of the noise.
Good luck...
Peccavi, March 2010
