According to the National Cancer Institute, a death rattle sound may occur when saliva or fluids build up in a dying person's upper airways and throat, and the person is too ill to clear the throat. One kind of death rattle sound happens with saliva in the back of the throat, and the other type occurs with airway fluid from a tumor, too much fluid or an infection.
About.com describes the sound as loud and rattling. It is a sign that death is imminent in hours although death can also take a few days. The website explains that any effect of the death rattle on a patient is probably minimal. To decrease the sound or extinguish it, a patient can be turned on his side to help clear the airway, and elevating the patient's position in bed helps drainage. It is also helpful to cap the amount of liquid that is put in patients' mouths, such as by removing excess water from sponges. Some medications, such as atropine, also get rid of extra secretions.
The National Cancer Institute also recommends a suction tube to suck up any excess fluid--but only if the cause of the rattle is fluid in the throat. If the cause is fluid in the airways, a suction tube leads to undue stress on the patient.