A dog may chatter his teeth together, or his jaw, occasionally or more frequently than you would like him to. Several reasons why this could be happening are:
Neurological Disorders
Your pet may be having neurological issues, such as seizures. There are seizures that are not very noticeable, known as partial seizures. These types of seizures can make the jaw chatter. Other neurological conditions may also be causing your dog to chatter his jaw.
Anxiety
Your dog may be overstressed or become nervous easily, and may chatter his teeth together or his jaw. Anxiety can occur due to a stressful home environment, a thunderstorm or fireworks, other loud noises, or when around unfamiliar dogs.
Cold
If your dog is cold, his teeth may chatter. Just as humans sometimes do in frigid temperatures, dogs that are out in the cold may have difficulty tolerating the temperature drop. If this is the case, you may not need a veterinary visit, just be sure to keep your dog warm.
Dental Condition
A dental condition may cause your dog to not want to bite down on his teeth or chew on food. He may also chatter his jaw as a response to the pain from a tooth abscess or gum infection. If your dog is a puppy, he may be teething and may also chatter his jaw.