My maltese Valentine (who passed tonight) used to do this as a little one. The vet said that the maltese can be a very stressed out, high maintenance, or nervous dog and that's why she was doing it. You could always take him to the vet but as far as I know they don't make doggy bite guards so you may be stuck with the grinding or he may grow out of it...
Thank you for your reply but, I have searched his mouth and teeth closely. He only pops his mouth or what ever he is doing once he settles in to go to sleep. It is a strange popping sound.
Are you sure he DOESN'T have something stuck in there somewhere? Puppies are notorious for chewing on anything they can find. Is it possible that he picked up something to chew on and got it stuck in his mouth? It's common for them to get things caught BEHIND the teeth, across the roof of the mouth, and it might be that he actually does have something stuck in there. Have you checked his mouth thoroughly?
Ghilly