death rattle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gurgling or rattling sound sometimes made in the throat of a dying person, caused by loss of the cough reflex and passage of the breath through accumulating mucus.

from The Century Dictionary.

• noun A rattling sound sometimes heard in the last labored breathing of a dying person.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The raspy or gurgling sound sometimes made by a person as he or she draws in or expels the final breaths in the moments before dying.