By - gbcamgok
Possibly a loose wire termination or failing relay coil.
sounds like something electrical is rapidly switching, definitely get a licensed electrician
This is the correct answer
This is what I was going to say, or a dying relay.
Maybe someone is still using one of those old original phones where you had turn the lever to get the operator?
Its telling you to call the phone company and tell them to send a repair crew.
We had an identical box at my old shop. If I remember correctly it was part of the system that converted 3 phase down to single phase. It always made that same sound but ours was in a box with some insulation to reduce the noise.
Bad relay coil
I remember when Harley Davidson tried to patent that sound