It sounds like the contented "tooth purring" rather than grinding then!
I hear that version a lot and 'touch wood' haven't heard the in pain version yet.
Bunnies can be so confusing, the first book I read said if a bunny pushes you with his nose it means 'go away', Artie did that a lot and it meant 'carry on stroking my nose', completely opposite to what the book said, luckily I worked out he was being sociable and not stroppy!