I've only known that I'm celiac since September 2010, but I have been coughing up phlegm chronically for a long time. It's definitely plausible that it could be celiac-related because of the overreactive immune response. I also think coughing up phlegm is somewhat of an athletic thing. I'm a runner, and when you're running it's a lot easier to clear your sinuses by coughing it up than blowing into a hankie. So i wonder if perhaps this habit has crossed over into my non-athletic life. That said, I definitely have a lot more mucus to cough up than the average person.