For some reason, the boiler can't disperse the hot water that it is generating - hence the rumbling noise of the water boiling before it shuts down. Some things to check....is the circulating pump running? Is there a wheel valve that connects the flow and return pipes together (the bypass) shut?
Are the radiators full of air? Has the boiler about 1 bar of water pressure? There may also be an air bleed valve in the top of the boiler - on one of the 22mm copper pipes usually - that should be bled. Try operating any zone valves by hand to see if hot water surges through into the radiator system.
John
June 2008