Noisy worcester 24i junior boiler - is it the fan or the pump?

I have a noisy boiler (which I have posted on before!) and I want to replace it but can't afford it! The noise I believe to be the fan - I can only describe it as a loud whirring sound... How can I be sure it's the fan and not the pump and is it worth replacing? I've heard that these boilers can get problems due to the heat exchanger getting covered with hard water deposits - and replacing the fan might not be a long term solution - any suggestions please?