why does high tension wires produce humming sound ?

The audible noise emitted from high-voltage lines is caused by the discharge of energy that occurs when the electrical field strength on the conductor surface is greater than the 'breakdown strength' (the field intensity necessary to start a flow of electric current) of the air surrounding the conductor. This discharge is also responsible for radio noise, a visible glow of light near the conductor, an energy loss known as corona loss and other phenomena associated with high-voltage lines. "The degree or intensity of the corona discharge and the resulting audible noise are affected by the condition of the air--that is, by humidity, air density, wind and water in the form of rain, drizzle and fog. Water increases the conductivity of the air and so increases the intensity of the discharge. Also, irregularities on the conductor surface, such as nicks or sharp points and airborne contaminants, can increase the corona activity. Aging or weathering of the conductor surface generally reduces the significance of these factors. "The higher voltages at which modern transmission lines operate have increased the noise problem to the point to which they have become a concern to the power industry. Consequently, these lines are now designed, constructed and maintained so that during dry conditions they will operate below the corona-inception voltage, meaning that the line will generate a minimum of corona-related noise. In foul weather conditions, however, corona discharges can be produced by water droplets, fog and snow." read less

Two reasons: One is coronal discharge. The high voltage between adjacent wires (phases) is enough to make air conduct. As the voltage between phases is alternating, and constantly changing, the sound changes. Second reason: Current through the wire isn't fixed. The varying current causes expansion and contraction of the wire, producing sound. Especially of concern is power factor far from 1, with current much higher at the peaks. Most electrical transmission is done three phase. So at 120 Hz and 120 degrees apart, each wire will expand slightly, creating a click, and the effective frequency might be as high as 480 Hz, explaining why it might sound higher in pitch. read less