AC hum and bounce of contact in magnetic contactor is due to mechanically damaged contact and variation of terminal voltage.
AC Hum in magnetic Contactor
Pollution of the magnet pole surfaces will create a slight hum coming from the contactor in the closed position.Hum can also occur if the pole surfaces are deformed. A louder hum will occur if the shading coil is broken because the magnet will cause the contactor to chatter.
If so the complete contactor needs to be exchanged. Another reason for AC-hum could be corrosion caused by environmental conditions that exceed the contactor specifications. Contactors need to be protected from condensation in order to keep the pole surfaces of the magnet free from corrosion.
Contact bounce in magnetic contactor:
Lower voltages will decrease the speed at closing. Both these factors can lead to a higher level of contact bounce at closing. High voltage and the increased speed at closing also contribute to some increased sound.