When you say "clack her teeth" do you mean she is chattering her teeth with her mouth closed, or she is biting the air? Both situations can be similar depending on the circumstances or dog, but each is a distinctively different behavior.
Teeth chattering with a closed mouth is typically a behavior dog's display when they are experiencing excitement, such as during play, or when they are happy you have come home - the situation depends on the dog. Male dogs may also chatter when they smell, or lick, urine or secretions from a female dog. It is actually quite common in male dogs. It has not been confirmed, but I feel this chattering behavior may be similar to the flehmen response, which helps mammals decipher pheromones.
On the other hand, biting the air is simply a sign of (usually bratty) excitement in younger dogs who want another dog to pay attention to them. It's kind of like when a dog barks in another dog's face. Playful, but kind of rude :)