The "on and off" of the solenoid is very rapid, that is what causes the vibration or what we perceive as the horn blowing it trumpet sound. I agree with the other members, clean it of rust, confirm that the armature is free to move, it should get better with GENTILE cleaning. You are in a better situation than I, mine won't hum, no heating of coil, I have continuity but don't know why it won't buzz.
