The most likely culprit is inductor windings. The magnet wire used for windings is generating a changing magnetic field which causes them to vibrate, usually at

The most likely culprit is inductor windings. The magnet wire used for windings is generating a changing magnetic field which causes them to vibrate, usually at the fundamental frequency of the excitation signal. (see image at http://www.digikey.com/product-detail/en/744773122/732-1258-2-ND/1638663 http://www.digikey.com/product-detail/en/744773122/732-1258-... for an example.) This is a fairly common source of audible noise in electronics.