Ahhh, quite the contrary, at least for a motorcycle like a Goldwing, they are whisper quiet. Some cars are extremely quiet too, especially ones like hybrids.

The reason I even bring this up is because I was riding my Goldwing one time when I came up on a horse rider, in my case heading the same direction as I was. I hesitated to pass because I didn't want to spook it. The rider turned and shouted to me to pass, as I did so the rider waved for me to slow and she suggested that I NOT stop like that again, not only was I creating a hazard to traffic behind us but the horse might actually get MORE spooked if it happens to hear stuff like tire crunching on gravel behind it for a long while. I asked if I should honk the horn, she said definitely NOT to and to just wait till it's safe to pass and then do so just like any other traffic. The horse trainer at the place we ride horses said pretty much the same thing as well.

Again, being that the rider in the OPs case was already OFF the road 10 to 20 feet plus whatever distance the bike rider was in on the lane it is unreasonable to require anyone to indicate they are passing, especially when the horse and rider are facing you. And YES, horses have the right of way, as do pedestrians, and WE have the right of way over cars in most situations.