Re: Why do transformers hum ?

They do have a noticeable hum.  It's due to the constant reversing of the 
AC current - 60 times per second in the US and 50 elsewhere.  The 
transformer core and case actually are moving, imperceptibly at that 
frequency, and the hum you hear is composed of harmonic frequencies of 
that 60 cycles per second, which is referred to as 60 Hertz or 60 Hz.
There are other harmonics at  120 Hertz (Hz), 180, 240, etc. with a decrease in the 
amplitude as the harmonic frequency goes up. The 60 Hz, 120 Hz, and 240 Hz
harmonics are pretty close to the "B" note on a keyboard or other musical instrument;
the 180 Hz third harmonic is close to an "F-sharp".

The hum sounds the same no matter what the voltages, in and out, of the 
transformer are, but for a low power distribution system transformer that 
you see mounted on poles, there is still a hum, but its amplitude is so 
low it can't be heard.  Actually, if you begin to hear it, something is 
wrong.

Large DC transformers are really AC.  For large DC transmission lines, the 
incoming power is AC and is transformed as AC, and then rectified before 
transmission.  So large "DC" transformers hum for the same reason as their 
AC counterparts.