Hi there and thanks for your question. There are a couple of reasons that could account for this behaviour.
If this is something that has never happened before and now is happening regularly it could be that there is a source of pain in your dog's mouth. She may have a tooth infection. Sometimes this can be obvious to see by looking in her mouth and feeling around her gums for pain, but sometimes it is not.
If this is something that she just does occasionally it may be that it is a behavioural things. Some dogs will chatter their teeth as a sign of sexual excitement. It is more common in intact animals but can still happen in spayed or neutered animals. They will do this more often if they have smelled something very interesting to them.
It is certainly possible that it is the tooth. It is really hard to determine if a tooth is sore...even when a vet is doing an exam. You can try pushing on the tooth from every angle to see if you can elicit any pain. I would also mention it to your vet next time she is having an exam and they can see if they can elicit pain as well.