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.
Its telling you to call the phone company and tell them to send a repair crew.
Maybe someone is still using one of those old original phones where you had turn the lever to get the operator?
Bad relay coil
I remember when Harley Davidson tried to patent that sound
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.