It's not until after she dies that Scout and Jem get a sense of what's going on behind the drool and venom: Mrs. Dubose is a morphine addict who had vowed to go clean before she died, and enlisted Jem and Scout (without their knowledge) to keep her off the stuff for longer and longer periods of time.
http://lungsounds.net/rales-lung-sounds/ - Rales lung sounds are small rattling noises heard around the lung. This happens when air exposes closed air spaces. Rales can be shown as wet, dry and coarse.
It is the crackling sound heard with a stethoscope when placed over the lungs. It is hard to describe it. If you google it you may find how it sounds like but a close one would be the sound you hear when you rub your hair close to your ears. It is mostly abnormal. There are many lung and heart diseases which cause it. It is caused by fluid or inflammatory exudate in the alveoli or bronchioles. It could also be caused by fibrosis or scarring of lungs which has many causes. Broadly the causes are infections, fluid in the lungs, cancers, inflammations, etc.