My customer has a Nortel 616 feeding to a newer Viking PA2A amp. The audio out goes to a Bogen TPU-15A and on the Bogen com and 70v screws to the two horns. Viking also has co lines to modular ports for ringing. When they page it gets a hum that sometimes lasts after handset hung up. When a call rings in it hums as well, sometimes a minute or so after call is answered. Does anyone have an idea on this. Thanks Dave.