Boiler becomes vey noisy?

I have a British Gas Condenser boiler which starts making a noise which gets louder then stops duiing the heating process. The engineer is about to change the pump overrun timer as the pump is contiually going even though it is not required. The engineer also serviced the boiler having made some adjustment. Unfortunately it never make the noise when he visits. Has anybody any idea what it could be. Sound like a combustion problem?

Phil E
October 2008