Both of my bunnies will do it when they’re sitting out in the living room and are feeling particularly relaxed. I know Otto doesn’t have teeth problems, so it doesn’t worry me. However, after Hannah’s recent molar spurs I keep an eye on her for signs that they’ve returned. I’ve been told that mild grinding can be an early indication of teeth problems – the teeth don’t hurt yet, but the bunny is grinding to kind of feel the points on the molar, if that makes sense.

If these teeth grinds are predictable, like you describe, I would not worry. Interesting what Elrohwen posted about the rabbit trying to feel the points on his teeth if he has spurs. I wonder if there is a certain sound to that kind of grinding.
Lint chatters his teeth during nail clipping and it is a threat in my understanding.