Transformer Humming

Humming is a sound which is produced due to the vibration of the cores in the transformer. The vibrations are produced due to the change in polarity of an alternating current. When the current is in its positive half all the cores of the transformer are magnetised with the same polarity and this produces repulsion between the core ends and hence core expands at the ends. When the current decreases to zero the cores attain their normal position. This happens four times in a complete cycle and produces vibration at the ends of the core and this results in noise.
This noise can be decreased by tightening the core of the Transformer

hi, actually transformer humming is due to a mechanical vibration in transformer core.An alternating current changes there polarity, some forces is acting and making a noise in lose tightining core.

Silly people. It's caused by the electrons at the end of each cycle bumping in to each other when they change directions.

insulting ?

Insulting? No. If i were insulting you, you wouldn't need the question mark. There would be no doubt in your mind.

It might be all the same reasons as computer noise. See http://cr4.globalspec.com/thread/58350/Computer-noise.