Eye witness reports all seem to state that they heard bangs and then saw the rotor separate from the helicopter. Given that it would take half a second or more for the sound to reach the ground, it would seem likely that the bangs might have been a prelude to the rotor separation.
So what caused the loud bangs? An engine compresser stall, the gearbox starting to separate and rotor blades hitting the fuselage, suspension tie rods failing sequentially or some other cause?
Given that the sounds are described as loud bangs, it is unlikely to be the sound of metal failing directly, more likely engine related or the sound of impacting rotor blades as some internal brace or the gearbox housing failed.
The Norwegian investigators should be commended for allowing the press so much access to the wreckage and for the open nature of the investigation. That may upset some in the industry but there is a tremendous amount at stake and it is important that there are no allegations of collusion or cover up when the investigation is complete.