Tinster and angelfj are on to the source and the cure for what ails your horn.
I had a similar problem after first getting on the road after resto.
Make a sharp enough left turn- the horn would beep. (Kinda embarrassing actually.)
Anyway, it was the slip ring contacting the 'horn brush' in the hub. These pieces need to be parallel. Wear and time take their toll on that relationship.
I rebushed the steering shaft with specially made delrin bushes (made by a machinist friend) to remove slop. Then I cobbed a special 'horn brush/horn button contact brush', not sure of the correct nomenclature, so that the default distances between brush and slip ring were correct; that is they didn't touch unless I pressed the horn button.
Has worked fine since then.
Sorry, no pics but Vol II of TRF parts book has pictures of these parts.
Keep on.