It's almost as if they're speaking in code. They don't want to spread panic, but they also can't hide the reality too much from those who understand engineering.

My theory is that the vessel pressure vented into the suppression pool which caused the drop in pressure. That could have made a loud noise, perhaps one they had never heard in testing. Since the pool is a metal torus containing water held up by concrete and tightly coupled to the other parts it's a rigid, massful, structure. It may have been damaged by the quake.

If the radiation is explained by the spent fuel fire at unit 4, the question remains whether or not the containment on unit 2 is still effective.