What causes the humming sound in transformers?

[woollymac]

Why do transformers hum? Is it the iron core vibrating & if it is what is it that makes it vibrate? Is it the little atomic magnetics in the iron changing directions with the ac? Can anyone enlighten me?

 

[Dadface]

Basically I think you are right,the coils carry a.c.and this results in the core going through many magnetic cycles per second.The changing magnetic flux then results in vibrations.

 

[Bob S]

Magnetostriction in the iron causes the iron laminations to change dimension when the excitation exceeds about 1 Tesla. Magnetostriction occurs twice per cycle, so the hum is 120 Hz (or 100 Hz) depending on where you live.