noisy fan ?

I have a worcester 28si combi and there is a noise when the hot water demand increases. Its like an increase in revs of an engine and the noise quietens somewhat when the demand is switched off again.
i can hear like a fan whirring going faster then slower depending on hot water etc. Im hoping this will be just an annoyance over the next few days to get christmas over before i call out an expensive fitter. Is this condition likely to deteriate and break down

rob smithson
December 2010