Wonder if anyone can help me? Install boiler 2003 been running well apart from replacing pc board 7 Electro spark leads, now for some reason we have this whistle type noise when the boiler goes to cut out(otherwise reaches temperature).Before it cuts out we gt this loud whistle/hum type noise that also vibrates down the pipes. have adjusted auto bypass valve to fully open. Also bled C/Pump & Rads. the system is open vented with 35mm flow & return pipes. the noise seem to be generated around the top left of heat exchanger. does this cleaning out? as there no oveheating banging noise before it reaches temperature to cut out. There is also fernox in the system.