May well be teeth. Poppy grinds her teeth but it's a 4 year habit. She only does it when she is getting comfortable now so she isn't grinding in pain any more
Our bunny Leo has some small sharp spurs on the one side of his teeth which need burring. This will be about £80, which was what Poppy's cost but her teeth are misaligned and she has abscesses on her jaw.
I would definately get the teeth checked though. A maybe £30 consultation and pain relief with more hay to try and help it is a lot nicer on the eye than £200 + (or then at least you can prepare yourself, I don't know what your vet prices are like
)
Edit: Re read your post- Poppy had huge abscesses but her 2 courses of antibiotics helped no end. She has top up courses every now and then to keep them small as they will always be there.