humming of a mobile phone charger
Transformer hum


  Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization.


The hum goes away when you plug your phone in because you've changed the load on the transformer. The phenomena is variable; some wall warts hum only when something is pulling power from them.

The hum likely means your charger is constantly using a couple watts, just to make noise.
The cure is to get a different charger. Some are better built than others. Apple and Nokia make/made chargers that 1) don't hum 2) aren't watt vampires.