I Have Had A Dry Cough For The Last 7

I have had a dry cough for the last 7 months. I have been to my primary doctor several times. Now I am seeing a pulmonary doctor. Rarely do I actually cough any phlem up but when I do it is like foam and white. I have had a mri on my sinuses and ok. Chest xray and ok. Booth breathing test and ok and a walking test which was ok. Do you have any suggestions?