I fixed this very problem on my '73 Road Runner this past weekend. The snap ring on the colum bearing at the turn signal was lose as it had not seated properly, this allowed the spring at the lower end of the colum to pull the shaft down just a little. I think the lock wheel was occasionally was rubbing the housing thus grounding the shaft which caused the horn to beeb when I turned the steering wheel. Good luck and be sure to let us know what you find. By the way the steering is much smoother now.