HH56 wrote:
Dirty contacts inside the horn could be at fault or rust could have settled in and the diaphragm has frozen up so it cannot move. Contacts are fairly easy to clean. Getting rid of rust not so much. In at least one horn I know of some kind of 6 legged critter had set up housekeeping way up in the air chamber and completely blocked the diaphragm movement.

There is a bit on the horns on page 49 in the electrical section of SM. You might try cleaning the contacts first. They are the item in the photo directly adjacent and slightly below the two wrenches where the mechanic is shown adjusting the tone. If that does not work then try to find out what is blocking the diaphragm. Contacts must be closed when power is applied so diaphragm can move and open them. Once that action starts the continual on/off of contacts and vibration of diaphragm is what makes the noise.