Chronic Bronchitis: This is a likely diagnosis given the patient's age and symptoms of a persistent cough with white phlegm. The lack of relief with acetylcysteine and no known exposure to irritants suggests a chronic condition rather than an acute infection or exposure-related issue.

Asthma: Although the patient does not mention wheezing, asthma can present with a chronic cough and should be considered, especially if the cough is worsening at night or with exertion.
Gastroesophageal Reflux Disease (GERD): GERD can cause a chronic cough due to acid reflux irritating the airways. The presence of white phlegm could be related to the body's attempt to clear the irritant.
Postnasal Drip: This condition, often due to allergies or a cold, can cause a chronic cough as the body tries to clear the excess mucus draining down the back of the throat.

Lung Cancer: Although less likely, lung cancer is a critical diagnosis not to miss, especially in an older adult. A persistent cough that does not respond to typical treatments should raise suspicion, even in the absence of known risk factors like smoking.
Pulmonary Embolism: While the presentation might not be typical, a pulmonary embolism can cause a cough among other symptoms. This is a life-threatening condition that requires immediate attention.
Tuberculosis (TB): Although less common in some areas, TB can present with a chronic cough and should be considered, especially if the patient has been exposed or has risk factors such as immunocompromised status.

Bronchiectasis: A condition characterized by damaged airways, leading to a chronic cough and mucus production. It's less common but should be considered if other diagnoses are ruled out.
Cystic Fibrosis: Typically diagnosed in childhood, but mild forms can present later in life with respiratory symptoms like a chronic cough.
Sarcoidosis: An autoimmune disease that can affect the lungs, causing cough among other symptoms. It's less common but should be considered in the differential diagnosis of a chronic cough.