What is causing the intermittent loud buzzing from my boiler?

We have a problem with a wall-mounted Ideal Classic combi-boiler installed in November 2004. About two months ago it started to make an intermittent very loud buzzing which lasts anything from a few seconds to a couple of minutes. Sometimes the boiler can go for hours without doing this, and at other times the buzzing is repeated two or three times an hour. It's very sporadic and irregular but the noise is loud enough to be heard from outside the house.
I wonder if anyone can explain this. Surely it must have happened elsewhere.