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Some horses do make a rattling noise when they exhale. A horse that is a "roarer" makes a distinctive roaring noise (hence the name of the disorder) when inhaling. This is a condition that can impair her aerobic capacity at high levels such as racing or galloping but should not be a problem unless it is very very severe if she is a true roarer. This condition can be treated (if severe enough) by an operation (tie back procedure(. An exhale noise is heard in many horses as they warm up, like a "snort, snort, snort" that corresponds with their footfall pattern and breathing pattern (they are linked patterns). I would watch her being ridden and perhaps video the session then you can determine when she is making the noise specifically. As she gets more schooled, this may disappear or at least dissipate somewhat, but some horses are just noisier than others. My own show horse was known to snort when cantering at shows in the warm up when he saw all the jumps but was quiet at home with or without jumps in the ring. If she is very soft from being off or unridden for the most part, this may also disappear with some fitness. Take it easy as she acclimates to the new work program, it will make it more enjoyable for her as she is fit up. I trust you have experience with green horses since you have taken quite a job with a 6 year old green Arabian, but if not please do work with a trainer for her safety and yours. Keep me informed as to her "noise levels"! As I said before, unless this is associated with inhalation it is more of a noise maker than a concern. Best of luck and let me know what you find out when you ride and video her while she makes her noises!
Susie L, MS