Could he have just eaten a cecal?
I’d keep an eye on him and make sure he’s not exhibiting any other pain signals, and maybe next time you pet him see if there are any sensitive spots.

My bun does that too. I really don’t think it’s anything to worry about. My little girl will binky all over the place before flopping down somewhere and doing the chewing/tooth purr to herself. She’s obviously in a good mood so I don’t give it much thought. When she does it though, it is more of a chewing motion than a normal purr. Kinda looks like how cows chew their cud.
But unless you are noticing other signs that he might in pain I wouldn’t stress about it.

When a bunny tooth-grinds from pain it’s not a subtle, hard-to-hear-sound, It’s a clattering sound and the bunny does not look happy. That’s how my bunnies have sounded when they’ve ground their teeth, both after their neuters and Bam when he’s had stomach-problems and I’ve been rubbing his belly.

When my Yohio eats cecals he’s sitting up, so it could be cecal-munching you’ve seen. If there are no other signs of pain, it probably isn’t pain, as bpash writes. But it’s good advice to check for tender spots when you pet him.

Bindi does something similar sometimes when he is laying down, like a little soft chewing motion. For him, it seems to be a sign of contentment.

The distinguishing differences between purring and grinding are that purring is a soft, gentle sound and most bunnies do it in waves (purr purr purr, pause, purr purr, pause) grinding is a louder, chattery, crunchy sound and it’s not done in waves like a purr, it’s more constant, like 30 seconds or so of constant chatter.

Hey, my Moss, who is 13 weeks has started to teeth grind (gently; she isn’t in pain) and ‘buzz’ when she is moving about… happy or starting to get hormonal? She is being ‘fixed’ in 6 weeks.
Thanks a lot.