Delete comment from: Ken Shirriff's blog
The physical vibrations come from the wiring of the inductor or transformer. Current moving through a wire creates a coupling force on the nearby wires pulling them closer and then pushing them away, when the current stops flowing at the end of the cycle. The wires displace a tiny amount on each oscillation of the drive waveform. Given that the coil has many hundred turns of wire, each of which, is displacing, the sound pressure generated by the movement can amount to something noticeable if it is in the audio band. This problem usually gets more noticeable as the power supply ages. Manufacturers coat the coil with glue to reduce the movement of the wires. As the component ages, the glue becomes more brittle and may crack or otherwise yield allowing the wires to displace more freely.
May 20, 2012, 6:46:22 PM
Posted to Apple iPhone charger teardown: quality in a tiny expensive package