A 50-year-old male presents to the GP surgery with a chronic cough that he has had for the last 4 months. The cough is productive however he does not feel unwell with it and he does not complain of any haemoptysis or any weight loss. His observations are all normal. He states that he has experienced this productive cough for three consecutive years, both times for more than 3 months. What is your main differential diagnosis?