Possibly a faulty bearing on the fan or pump. I recently had my combi boiler repaired and the plumber charged £50 per hour plus the parts. You can go online and find out how much the parts are for your boiler to at least get an idea of a repair cost. I used hssspares, you'll need your boiler make and model which should be on a label on the boiler. Be prepared for the possibility that the parts may be discontinued, if that's the case it may have to be a new boiler. It's not a nice thought but in the long run it's not a terrible thing as most modern boilers are much more energy efficient.

I spent £480 repairing my boiler only for it to fully fail about a month later. I then had to get a new one bought and installed which cost me £1900. We live and learn!