This computer humming sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.
Whenever the voltage passing through a conductor exceeding a certain value the air molecules around the conductor got ionised. the ions moving in the atmosphere, they take energy from the supply so a hissing noise will occur.. this effct is called corona effect..
The HT lines causes humming because of corona effect.when the voltage of transmission lines increases too much to produce electric field of magnitude of 10^7 v/m.That is more than the electric field required to break the inter molecular bond of the air molecules.Hence the air particle around the lines get ionized and start moving freely.and during this movement several collisions occur and resulting a humming sound.