The mucous might be related to a chronic inflammation of a sinus. You might want to visit a Ear, Nose and Throat specialist who will do an X-ray of the sinus. If the inflammation is caused by bacteria, then a swab from the nose and throat might determine the exact antibiotic. The effect of Zyrtec might indicate that an allergic reaction to an unknown allergen occurs all the time, thus causing inflammation and increased secretion of mucus. Adequate allergic tests might reveal the allergen that causes the condition. Avoiding the allergen might completely solve the problem.