Jaw clicking & crunching

My cat is manipulating her mouth and jaw around. I hear a loud crunching/clicking noise. She is 9, this has gone on for four weeks. The vet sees nothing on exam, and thyroid test came back ok. Frequency of this behavior: several times an hour. She eats and drinks just fine. Behavior frequency is escallating.

Answer by Kate
well if the vet doesn't think that there is a problem then it must just be a habit the cat has started. I must admit to have never come across this before but if there is nothing physically wrong then you will just have to put it down to one of your cats funny little ways.