Has there been a thunderstorm where you are? This can sometimes be a sign that the dog has got worked up over something, and we have had one hell of a storm. They don't necessarily have to be frightened of thunder and lightning, but just the atmospheric pressure can set them off.
Of course it could be something as simple as a loose tooth
or something stuck either between his teeth or in the roof of his mouth.
Another common cause for males is when they smell a bitch in season, and they do a 'Hannibal Lecter' chattering their teeth in excitement.
If it doesn't stop then I would get the vet to check his mouth.