Why do geldings make a weird noise when they trot?

My friend told me it was because of their sheath, but I don't understand how it could be.
Also, what is a sheath, I know it's something on the horse's penis but idk about anything else.
And do stallions do it or is it just the geldings?
Update: Majke the noise.
it's like if you made an "uh sort of moise in your throat, kind of like your swallowing.
It's hard to describe, and it's rythmic, not like a random pattern of noise.
Update 2: Yeah, it sounds like it's coming from their hindquarter area.
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