Why does our Worcester-Bosch boiler all of a sudden? It's 18 mths old?

We got our Combi Boiler installed by B Gas only about 18 months ago and the warranty has now expired. We don't understand why it's only making the noise when it seems to be heating radiators or water? Everything is functioning ok it's just an awful loud vibrating noise. No pressure loss, radiators all bled and ok.