Practically all bunny behavior must be interpreted in context. A bunny can grind its teeth in pain – that gives off a chattering sound and the bunny doesn’t look happy. It may sit all hunched up or be pressing its tummy against the floor (active pressing, not just lying on its belly like they do when they relax). A bunny in pain is often withdrawn and doesn’t come hopping up to you to beg for or take treats from your hand etc.
Happy bunnies also grind their teeth. That’s not a chattering sound but a softer one, more like a purring (it’s sometimes called a tooth-purr). Sometimes you don’t even hear it but you can feel vibrations if you touch the bunny’s head.
If there are no signs of discomfort, if your bunny eats, drinks, poops and socializes like normal, I’d think this is a happy sound.
(If on the other hand she’s drooling while she’s doing it she may have a tooth-issue. There should be no drooling whatsoever and eyes should be clear without disharge.)