Why is this downvoted, this is a very reasonable question.
Noise is a mechanical wave produced by something vibrating.
If you can hear it, it means that thing is vibrating quite hard inside your monitor.
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-... for an example.) This is a fairly common source of audible noise in electronics.