What to check

Check whether there is a coded access unit, door panel lamps or a third party device is being powered from the system's AC output or is sharing a common conductor to the power supply unit with phones or the door unit's amplifier module.

Confirm that the hum stops when the AC fuse in the power supply is removed.

Corrective Action

Consider using DC voltage instead or use a dedicated conductor for '0' to the power supply.

It is good practice for third-party devices to be powered by their own dedicated power supply