Transformer hum is primarily caused by magnetostriction in the core. As magnetic flux reverses each cycle, core laminations slightly expand and contract, creating mechanical vibrations at twice the line frequency (100/120 Hz) and harmonics. Loose clamping, aging insulation, and structural resonances in the tank, radiators, and mounting structures can amplify the audible noise. High load levels and harmonics further intensify the vibration. Proper core clamping, damping materials, and acoustic design are used to reduce hum in substations.