I've had to work on accouple ships horns in the past (usually while stuck in the fog DownEast Maine, apparently thats where they all break).
Any way if its just a buzzing noise it sounds like the horn is not getting enough air. So if its not the tube connections (try the soapy water trick, to see if air is coming out), it could something is lodged in the horn itself. What happens if you blow on the horn part seperately? Also can you feel air coming out of the compressor? I would imagine you should feel some air movement with that compressor
Good luck
