Every time you read about an executed inmate's final moments, the report includes "s/he snored." So I did some looking. Turns out they aren't really snoring. It's the sound of air escaping as their diaphragm and lungs collapse when the muscle relaxant goes to work. My source was an interview, the transcript of which is here, in the Texas capital punishment thread. A better description, that would probably set some peoples' hair on fire, would be "death rattle."