Once Baxter was chewing on an antler and then he started doing something similar to what you're describing. We thought he had something in the back of his throat that he couldn't get out. After a sweeping check of his mouth, we found that was nothing there. I took him to the emergency vet and the vet said he might have scratched the back of his throat. When we got back in the car, he stopped making the chewing motion.
Is this something that he just started doing or something that he does occasionally?
My most serious thought would be a petit seizure...if it keeps up I'd take him to the vet...otherwise if he's young I'd think a loose tooth or something stuck between his teeth (rawhide?)