Hi S.,
My daughters did that when they slept, and I agree, it is a horrible sound.
The dentists told us the same thing, that they would more than likely grow out of it, but if they didn't, they'd have to look at mouth guards, and braces, that type of thing.
The did eventually grow out of it, can't remember exact ages. My youngest still does it sometimes, but not nearly as bad.
Makes you want to grind your teeth too though, doesn't it, when you're hearing it?
If yours does it when she's awake, maybe give her something nutritious to chew so her mouth is doing something else. Celery, apple, carrots, cucumber, things like that.
I hope she stops soon, for her sake and for your sanity.
K. W