Due to the high voltage in the lines, the air surrounding the transmission line gets ionized. Due to this, the particles vibrate at high speeds as they have high energy. These vibrations together form the humming sound which can be heard by the people passing by. The reason for the humming sound is the corona effect.
The HT lines cause humming because of the corona effect. when the voltage of transmission lines increases too much to produce an electric field of the magnitude of 10^7 v/m. That is more than the electric field required to break the intermolecular bond of the air molecules. Hence the air particle around the lines gets ionized and start moving freely. And during this movement, several collisions occur and resulting in a 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.