That's mucus/secretions down in your lung. Don't freak, but don't disregard it. You may need an antibiotic to rid your body of whatever infection is that is causing swelling, inflammation and secretions. I got bronchitis and coughed and hacked for days, refusing to go on an antibiotic due to side effects. When I started my morning coughing one day, there was a noticeable crackling or "rattling" at the very end of my exhale. I'd had pnemonia before. I was afraid. Others could hear it and I could feel it at the bottom of my lung. But it was loose and in my airway so I coughed until it was gone, but it was there again the next morning and then it began to increase over the next couple of days. Needless to say, I got on an antibiotic stat, and it's now gone... along with the bronchitis and most of my coughing! It may be what's called walking pnemonia which isn't alarming so don't freak out, but do get medical attention anytime you have congestion in your lung.