The same thing happened to my boiler this morning (same make/model).  It sounded like a kettle boiling and then the almighty bang woke the house up! Initially, I thought the boiler had frozen overnight due to the extremely low temperature out side, but when I reset the system I heard the pump kick in.  The first thing I did was bleed the system of air - but I need to keep an eye on the pump.
                 Koola
                
 November 2005