GP 38-2 in Youtube video - penetrate background noise

Hello
what part of the engine makes this penetrate background noise in this video :
http://www.youtube.com/watch?v=SJ0THFpVF0I
You can hear this humming almost the hole length of the video ...

Is it the traction motor blower ?
Or does the horn value leak and a little bit of compressed air flows permanently to the horn and makes it "honk" continuously?

Greetings

Good question! I've never heard any GP38-2 sound like that before. I like your theory about the horn valve being stuck partially open... that could very well be the problem.

does sound like a leaking horn valve, as the sound changes when the horn is blown. it actually quits each time the horn stops blowing, but leaks back on right after. sometimes you can tap the valve with a spike or hammer and clean off the seat. other times it's damages, and you cant do anything, but try to ignore the loud buzz over your head, and report it.