My boiler has started to make a very loud knocking noise when the central heating is on. The boiler is 11 years old and is regularly serviced. The thermostat is also not operating above 21 degrees. I have been told the boiler needs to be replaced at a cost of £2,500, can anyone advise me if this sounds right? Also been told a new one must be a condenser type, the present one is a combination without the water cylinder. When the boiler was installed, the whole system was flushed through. The noise starts up after a few minutes and sometimes doesn't happen at all. Any answers or help would be most appreciated. Thank you, Dee