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.