Just a guess here, a vacuum leak causing a cylinder to lean out and run hot enough to detonate. You can find a vac leak, or at least narrow down the area by using a 3 or 4 foot piece of heater hose or something close to that size. (Section of the neigbours garden hose!) Hold one end up to your ear, and use the free end to listen for the whistle. The hose cuts down all the other noises that are going on, and concentrates on whats close to the open end.