Magnetostriction is an effect that causes ferromagnetic materials to vibrate under changing magnetic fields. The mechanisms of this effect are explained below:
Because a magnetic field exerts a significant force on ferromagnetic materials, such a material in a magnetic field will change shape slightly because of the stress acting on it. If the field changes intensity and/or direction, so will the intensity and direction of the stresses. Therefore, if a continously changing magnetic field is applied to a ferromagnetic material, there will be a constantly changing stress, causing the material to vibrate in step with the field.
This effect explains why power transformers hum. The frequency of the hum is usually double the frequency of the changing field, because each cycle acts twice to vibrate the material. Magnetostriction is put to deliberate use in ultrasonic transducers.