I have a new worcester greenstar 30si combi boiler. Recently, even though the central heating was off, the boiler started making loud noises in the early hours of the morning. The noise was like a vibrating, droning sound but very loud. The boiler has been working very quietly for three months and I've had no problems until now. Those noises stopped the next day and the hot water is working fine. If I switch on the central heating, the boiler makes a few small grumbling, droning noises over a few minutes, I can hear water inside the boiler, more than usual - then the reset light (fault indicator) goes red and the blue light flashes. If I reset the boiler, the same thing happens. If I disable central heating it is working fine. Pressure guage is at around 1.5 and there is no air in the radiators. The only thing that has happend recently is that I replaced the overflow pipe, just a screw-fit jobby so I havent taken off the cover or messed with the boiler in any way. Any ideas?