Here is an article I found.
What does teeth grinding mean? I think there is a horse in our barn that does it sometimes.
Teeth grinding is one of those things that is difficult to describe, but you know it when you hear it. The sound is quite unique, a muffled rhythmic grinding, louder than the loudest gut sounds. And if you look closely, you can see the jaws moving in sync with the sound.
Teeth grinding is almost always associated with physical pain or discomfort probably of a fairly high degree. Teeth grinding can be associated with pain of various origins, but is a fairly common with gastrointestinal pain (belly ache).
It has been suggested that teeth grinding can result from psychological or social stress in horses. If it does, it is likely fairly uncommon in otherwise healthy, pain-free horses. So, whenever teeth grinding occurs, we should probably look long and hard for physical pain and mention it to the vet.
Is the horse getting lots of forage (hay), Maybe too many supplement? Horse guts are pretty complex, so it could be a multitude of things. I'm not a vet so I won't even try to diagnose.