Humming Noise in Electrical Transformers and Appliances

[PhysicoRaj]

Every time I walk by the electrical transformer at the end of the lane, I hear a low humming noise(≈50Hz) that is continuous. I have observed this in all types of fans. I grab the hub so that it is not able to rotate and switch it on. Then I hear this hum. When the mixer/grinder chokes, it starts to hum in the same way, same frequency. What is the cause of this and does it hum in only AC circuits?
Thanks for any help in making me understand this.

 

[mfb]

Voltage is switching every cycle of the power grid frequency, which is 50 Hz in Europe, 60 Hz in the US and one of those values in the rest of the world. Transformers generate magnetic fields with that frequency, and magnetic fields can move objects - if there is anything which is not perfectly fixed, it can move a bit in each cycle, with a frequency equivalent to the power grid frequency (or multiples of that). Oscillating objects emit sound.

 

[PhysicoRaj]

Thanks. I understand now.