...On a deeper level, not being affected by such things is sincerely good practice for others who sit; they get opportunities to let go of each new sound.
You might talk to the person in charge of the retreat, just to be sure, but I agree with Toby. I have met a young man with Tourette's (quite noticeable and frequent physical and verbal ticks) who is a very competent horseman and rider (and a really nice kid). The horses he works with are the better for being in his care, as they learn to not be easily startled or distracted.
I'm sure it causes you daily frustration, but I'll bet others are not nearly so concerned about it as you might think. Telling them up front would alleviate any concern or distraction they might feel (for instance, wondering if you are upset, or have the hiccups, or whatever). The young man I referred to tells people when he meets them, basically "Hey, I have Tourette's, so I make weird noises and jump sometimes," and the response is universally along the lines of "Oh, cool, whatever. Nice to meet you."