There have been a few people who have mentioned whistling, ringing bells and making other unusual noises to alert the horse and rider.
Whilst I'm not a horse rider, I would assume that speaking/singing etc would be better, as this allows the horse to recognize you as a human, not some potentially dangerous funny two wheeled contraption making odd whistling and pinging noises.
It's as much for the benefit of the horse to know you are there and what you are, as it is the riders.
Someone with more experience can correct me if needed.
Whilst I'm not a horse rider, I would assume that speaking/singing etc would be better, as this allows the horse to recognize you as a human, not some potentially dangerous funny two wheeled contraption making odd whistling and pinging noises.

It's as much for the benefit of the horse to know you are there and what you are, as it is the riders.
Someone with more experience can correct me if needed.