If you mean the dog' s head is trembling in small, rapid movements, look at his eyes. If there are small, rapid but coordinated movements of the eye from side to side and if the dog is older then I would suspect peripheral vestibular disease. We've had two dog's so afflicted over the years. Vet's don't know what causes it, there are no treatments but fortunately the symptoms go away... in our case about two weeks or so. It sometimes produces nausea, since it's rather like you experiencing sea sickness or other motion related sickness. In any case, if it was rapid onset or if there are other noticeable symptoms, I'd certainly have a Vet look at the dog... Best of luck...