Follow-Up:
I have started to experience this problem in the last few months. It has happened about 6 times and I find it very alarming. I am suddenly awoken by a loud bang, like a gun shot, which I at first thought was external and even woke my husband to ask him what the bang was. By about the 3rd or 4th time I realised it must be in my head. I still find this upsetting and I have problems going back to sleep.