Horse Is Grinding Teeth In Rythym?

I recently purchased a new horse. She is a Registered Quarter Horse with a lot of Thoroughbred in her blood. She looks more like a TB too! She is 6 years old, and is broke. For a while now when I have been free longeing her, she has grinded (clacked?) her teeth together to the rhythm of her gait. She doesn't do it at a walk, mainly at a trot. Every stride she takes she will do this. Not to mention, HARD! I can feel it in the longe line itself not to mention hear, and see it! I'm probably going to have someone come out, and look at her teeth soon, but I wanted to see if anyone has had a similar thing happen, and what they did about it if anything. She doesn't seem to be in any sort of pain by the way. When she's done this she was always moving freely with her ears forward. And, she does not do this when she's being ridden either. Only when longed or in showmanship on the ground. Also, her eating habits are completely normal. She does, however, have a history of pacing fence lines, and making faces (pinning her ears back) when asked to do more than she desires. She has not paced for a while now, and gets softer with us everyday. But, has not stopped doing this. Any information would be VERY helpfull. Thank you for your time, and consideration. (: