Dry cough for more than 3 weeks. Help.

I am Aditya Wadkar, i am 29 years old. From Thane, Manpada. I am having dry cough from past 3 weeks. No fever, no cold, no running nose. Just dry cough. My family doctor had suggested me these medicines: 1. T. Ventidox M (5 days after BF and D.) 2. C. Pulmoclear (5 days after D) 3. Syrup. Digecaine (5days 10ml after L and D.) He also asked me to get HR CT chest dignosis which has come normal. the results are attached. But nothing seems to work. The coughing is persistent and it pains in chest when i cough. My doc. Suggested me to get an opinion of ent specialist. I have this dry cough problem since childhood. Every 2 or 3 years i get dry cough that lasts more than 3 to 4 weeks. Every time doctors suggest me different medicines but nothing works and eventually the coughing stops after 3 to 4 weeks on its own. Please help.