howling noise on start up what is causing it?

I have a Glow-worm 15hxi gas boiler for my hot water and c/h system fitted by British Gas approx 30 mnths ago it is running 7 rads with an eigth oulet on the manifold for another rad blanked off. On ignition from cold the boiler starts to make a howling type noise which builds as the system heats up when the rquired temp is reached and the burners shut off the noise ceases but there is a tiny similar noise constantly . Then when burners reignite to bring temp back up to reqd setting the noise repeats itself. I have had numerous visits from British Gas engineers who have tried a variety of things to attempt a cure ie . replace heat exchanger, renew fan, renew bypass valve in system, renew heat seals etc etc but still no cure. Any suggestions would be welcome as Brit Gas engineers seem unable to solve the prob.