One possibility is allergies. If the phlegm is clear, white or pale yellow, this may be what's causing it. I am dealing with the same thing right now. It came on all of a sudden last year (I'm 38), and I have it almost daily. I went to an allergist and had testing done. I've been on allergy shots for about 6 months and have seen slow improvement.
I have the same problem.I just went to the ENT DR. He did blood tests,and had me get a CT of my sinuses because mine always turns into sinus infections or an upper respiratory infection and I'm always on antibiotic. I have a constant drip in my throat and am always coughing it up. The Dr. said everthing came back fine except that I had an allergy to mold. He told me to see an allergist that I need a full allergy work up that it's most likely allergy related and to take antihistamine to dry it up so It won't turn into an infection. So I'm going to an allergist soon. It's environmental I believe because I never really had this until a couple of years ago, but it's getting worse. Hope this helps! TLove