A very common occurrence. The bushings in the horn frame have weakened and need repairing.. there are a few threads about that here and a really good one over on Rentech.com,
I think you'll have to register on renntech to see it but it is a good thread:
http://www.renntech.org/forums/index.php?showtopic=1728
Nothing personal folks, we are hear to help but, a quick search before posting is always a good idea. There are several threads about this here on the forum, here is a thread from just a few days ago about this very same thing:
Horn doesnt stop Honkin... 