I had someone round to service my baxi solo 2PF boiler. At the end, he referred me to a noise I hadn't noticed before - a constant high-pitched whine, that now never stops while the boiler is on. He said the noise was caused by dirt inside the boiler - he said that the boiler would continue to work for a while but that I might like to consider having a new boiler. I asked him how much it would cost just to get the boiler cleaned and he suggested about £370 because he'd have to use chemicals etc. Does that sound like a fair price? Is there another possible reason for this constant high pitched sound?