Tooth purring is the bunnies response to something nice eg. being stroked, It may be a quiet grinding of the teeth or chattering teeth.
When in pain some buns make the same grinding sound, but nothing nice is happening to them. It's louder, & they are usually lying differently from usual - perhaps hunched up lifting the tummy off the ground on their back feet, (tummy pain) or protecting a dislocated /painful joint.
My boy has to be different! & if he's asked to be stroked & got in position, he does 3 loud grinds to get my attention, "Oi it's time for a nice cuddle".