Why Do Gelding's Hips Creak?

So i've noticed this several times in the past but have never taken the time to figure it out... why do some gelding's hips "creak" when they trot?
I've only observed this in geldings and more than one. It only happens when they trot & it's sort of a creaking/grunting sound.

I heard it was air going in and out of the sheath. I don't remember where I read that though.

I have no idea, but I've always wondered what it was too..

I've also heard it's from air going in the sheath. But I remember someone saying their mare had that same noise too, so hmm...

Yea its always been weird to me lol. I ve heard its only a gelding thing. I can't wait to see what responses you get!
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Yup, not the hips. It's the sheath-squeak! If anyone's heard that in a mare -- I hope her name is Lady G!

Wow! I never would have thought that. I was wondering about this, too. My TB does this, but not my arab and I always thought it was a size thing or a weight thing. Crazy.

Interesting observations regarding "sheath squeak"--

Jessica Jahiel's HORSE-SENSE Newsletter Archives

Learning About Horse Riding Online: Sheath Noises In Geldings

Time for the W-D 40?! :p
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I have been told it means the sheath is dirty.