Power transformers hum primarily because alternating magnetic flux in the core causes magnetostriction, where the core laminations physically expand and contract at twice the supply frequency. This vibration is transmitted through the tank and structure and heard as a low frequency hum. Additional contributions come from winding forces and loose components. Designers reduce noise by using suitable core materials, clamping the core firmly, optimizing flux density and sometimes adding sound dampening enclosures. Excessive or changing hum can indicate over excitation, loose parts or abnormal operating conditions that require inspection.