The reports of 2 x loud bags would probably be the engines over-speeding or more specifically the free-turbines. With the head gone, there is no load, so they would run up and most likely experience a turbine burst.
I saw this in a Eurocopter demonstration video once and it is an explosion but contained within the reinforced engine casing. Not that the reinforcement would have helped in this situation sadly as bigger things were in play.
P3