Here is a video of one of the horses we trained in 2010. He was adopted by a girl in Mississippi.
You can get a horse to accept almost any kind of sound if you make the sound a predictor that food is arriving. Just switch it on (then quickly off) for a second and hand the horse a treat before it has time to react. Do that about 10 times. The machine makes noise and the horse looks for a treat. Then start leaving it on for longer and longer. Hey, you want your horse to come running when you get out the mane clippers? Not a problem. You can throw the treat in a dish if you don't believe in handfeeding.