Very difficult to diagnose. If there were other people in the house at the time you could ask them if they did anything to cause it.
The bang itself, what you heard, may have been an electrical arc caused inside (or worse, outside) a junction box. They do sound a bit like a book being dropped.
It may have been caused by damaged wires, foreign objects, physical damage, or other problems. The arc itself may have altered the problem so it doesn't immediately arc again. It may have improved the situation, for example, a stray bit of metal inside a junction box shorted the terminals, and was either incinerated or sent flying well out of harm's way. Or it may have worsened the situation, for example, damaged insulation on a cable exposed to a nail or pipe, and the arc moved the cable away from the metal object but also further damaged the insulation, which now is a bigger problem waiting to happen.
This is really hard to diagnose but here's what I would do:
- Think about any human activity during the prior week. Was any electrical thing added, removed, relocated, repaired, etc? Was anything hung, screwed, nailed to any wall, ceiling, or floor? That will provide priority for the search.
- Breaker popped but not RCD. That SHOULD mean there was a live-neutral short, not a ground fault. Not guaranteed but it may narrow the search a bit. The problem is likely to be in a junction box where live and neutral are both exposed, OR a nail clear through a cable shorting live to neutral.
- If there was any nail, screw, etc added recently ... start there. Be careful, it may be live (now touching hot but not neutral).
- If you've ever had squirrels/mice/rats in your attic inspect as much as practical for cable damage. If you have an open crawl space under your house, inspect exposed cables for rodent damage. Within reason (and with gloves and goggles) push aside insulation to inspect cables. Rodents love nesting behind insulation.
- Open up all the wall sockets, ceiling roses, spur fuses. If there was any human activity recently, start with those locations and then extend to everything on that breaker. If the arc was in a box it should be obvious from the soot, but look for other signs like foreign objects, loose cables, overcrowded boxes, etc.
- While each box is open turn the power back on and carefully use electrician's pliers to wiggle all the cables entering the box. Wear goggles. You're trying to provoke the short circuit.
If none of that reveals the problem it may be hidden inside the walls. There isn't much else that can be done. Change the batteries in your smoke detectors.