Pulmonary Embolism is unlikely but Bronchitis or other minor respiratory infections could cause this symptom due to inflammation and minor bleeding in the airways. This condition is common and can cause intermittent hemoptysis (coughing up blood) without other significant symptoms.

Gastroesophageal Reflux Disease (GERD): Stomach acid reflux can irritate the throat and cause minor bleeding, leading to spitting up small amounts of blood.
Pulmonary Infections: Infections such as pneumonia or bronchiectasis can cause hemoptysis, although they are often accompanied by other symptoms like fever, cough, and shortness of breath.
Allergic Reactions: Allergic reactions to environmental factors or substances can lead to inflammation and minor bleeding in the respiratory tract.

Lung Cancer: Although less likely without other symptoms, lung cancer can cause hemoptysis and is critical to diagnose early. It's essential to consider this possibility, especially in smokers or those with a significant smoking history.
Tuberculosis (TB): TB can cause hemoptysis and is crucial to diagnose due to its infectious nature and the need for prompt treatment.
Pulmonary Embolism: While less common as a cause for isolated hemoptysis, pulmonary embolism is a life-threatening condition that requires immediate medical attention.

Mitral Stenosis: A heart condition that can lead to hemoptysis due to increased pressure in the pulmonary veins, although it's usually accompanied by other cardiac symptoms.
Goodpasture Syndrome: A rare autoimmune disease that can cause hemoptysis and kidney disease, often presenting with other systemic symptoms.
Bronchial Artery Aneurysm: A rare condition where an aneurysm in one of the bronchial arteries can rupture and cause hemoptysis, which might be life-threatening if not promptly treated.