As Speedy Jim said these are 12 volt AC switches that operate the
relay contacts for the normal 120 volts AC. There is nothing wrong
with this type of system. It is safer and more flexible. A certain
amount of hum is normal because the solenoid coil that operates the
relay contacts gets magnetized by a 12 volt 60 cycle source coming
off of a transformer. If the solenoid buzzes to much then it's time to
replace the relay.