I believe one of the 15/6 orderers had a 6nd410 come broken from shipping. Might want to pop your jack plate off and shine a light on the back and see if there's any damage or anything peculiar back there.
That said I would be shocked if it was not the horn. Wiring up a horn cutoff switch would definitely be worth your time.
Yup, my 6ND410 had a crack in it (see pics below). It was noticeable at higher volumes, and it would buzz pretty badly. I noticed the cracks, informed Don, he arranged for a new 6ND410 to be shipped to me, I installed it, and all is right with the world now.
Link Removed
Link Removed

