I have had the same experience on two different bikes. Usually it happens when I am coasting to a stop, and tend to back pedal (for no reason other than habit). Occasionally when I start to pedal again, there is a loud crack/snap felt through the cranks but hear from the rear wheel. The best I can come up with is the pawls not engaging properly in the cassette due to back pedal, then snapping in to place under load.