I have had gastritis/GERD/IBS for years. BE produces clear, yellow and green mucous. GERD also produces clear sometimes foamy mucous.
To rule out a lung infection, your pulmonologist needs to order a sputum culture and MAC culture.
Sometimes I am not sure the coughing is from my lungs or stomach.
Most of my coughing is in the morning from lung. Later in the day is probably GI.
Good luck!