I Am Not Ill But In The Last Few Months

I am not ill but in the last few months I occasionally have a dry cough but it produces these small white, bubble filled formed pieces. They started small sized and now are the size of a dime. It seems to come not from the throat but from the bronchial area. I asked my lung dr. about it and he said he had never heard of it. The cough is not chronic but happens daily. Sometimes I swallow these white things so they don't always come into my mouth. I have well managed allergies - shots and meds so I don't think it is related to that. I do not have drainage. What is this stuff? Is there a need to ask my internest about it? Thanks, Betty