Transformer & Ballast Noise
Some noise is produced by low-voltage lighting systems from both the lamps (light bulbs) and the transformers, especially when dimmed. This noise can be audible depending on the lamp and transformer combination, the level of background noise, the absorbency of the surrounding surfaces, and the hearing ability of the people involved. Non-dimmed, magnetic low-voltage transformers will be audible to most observers. Use a Lutron magnetic low-voltage dimmer to reduce the humming. Electronic transformers produce less noise and are virtually inaudible. Use a Lutron electronic low-voltage dimmer to maintain this quiet.
All ballast systems emit some noise, which may be audible depending on luminaire (fixture with lamp inside) mounting and spacing. Make sure the ballast is mounted tight inside its housing; a loose bracket or screw could be the cause of excessive buzzing.