i believe it was probably a pack failure they are located in the centre of the aircraft where the bang came from hence condensation mixed with a bit of oil
As the a/c did not depressurise no oxygen masks needed. Also as the a/c landed safelyI with no fire the crew took some time to decide whether to use the slides, as there use is not without risk.
Lets see what comes out of the investigation but I believe the flight crew handled this incident well