So generally speaking displays--TVs or monitors--produce noise when they have internal power supplies. You will find anything 27" or smaller in general uses external power bricks and thus the noise is at the ground, much farther from you. Also some power bricks are well dampened so there is no noise. Internal power supplies make noise for a variety of reasons, either the transformer/coil windings are rattling, or the capacitors are vibrating/ringing. In general these can be DIYed away if you are willing to void your manufacturer warranty by opening up the monitor and either gluing down the PSU or replacing the capacitors.
Many high end TVs that use large PSUs--bigger than what is available in commercial external bricks--have very well dampened power supplies, but lower end TVs and monitors use cheap parts and no effort is made to keep the PSU from rattling.
The majority of monitors 30" and above use internal power supplies as they generally draw nearly 200W at peak. Anyone who owns a first generation Xbox 360 knows how large an external power brick has to be for 200-250W...to save on shipping costs most manufacturers simply slap it on to the TV/display so you don't need extra box space for a power supply.