Teeth grinding can also mean pain, but it's pretty obvious when it does - the bunny will sit pretty hunched up, looking off into space and appear uncomfortable. If she's moving around and checking things out, I imagine it falls into the "tooth purr" catagory. Some bunnies will only do this while being petted, but my bunny is known to do it when he's just sitting around being happy.
Has she been to see a vet? During a regular check up they'll look at her teeth and make sure there's nothing wrong that could cause tooth grinding.