I was in bed yesterday having a quiet read with my BS4/BL3 combo on quietly in the background when suddenly there were several very loud bangs from the speakers. I nearly jumped out of  skin. I quickly switched the system off thinking the system must have blown but I switched back on again and everything seemed to be fine.

This morning, again, they seem OK. I've got another 3 years of warranty to run so I'm not unduly worried - if it happens again I'll be straight onto my dealer but has anyone got any ideas what it might be? I was thinking maybe something to do with the digital radio transmission going haywire or an electrical surge. Any ideas? 

I am assuming that you were listening to DAB radio at the time and your "bang" could be a short transmission fault or more likely it could be a "Brown out" or "mains spike". If it does not happen again, then you can assume it is caused by one of these problems.

Brownout Definition: A reduction in the mains voltage without a complete loss of power. A brownout is a steady lower voltage state. An example of a brownout happens during peak electrical demands in the summer, when utilities can't always meet the requirements and must lower the voltage to limit maximum power. When this happens, computer systems can experience data corruption, data loss and premature hardware failure and Video and audio systems can have variable symptoms including speaker noise, shutdown/restart and switch off.

High Voltage Spike Defintion: High voltage spikes occur when there is a sudden, rapid voltage peak of up to 6,000 volts with a duration of 100mS to 1/2cycle These spikes are usually the result of nearby lightning strikes, but there can be other causes as well. The effects on vulnerable electronic systems can include loss of data and burned circuit boards.