Our boiler has started making a very loud whirring noise. Starts about 30 seconds after boiler fires up. After it has been running for a while it gets a bit quieter but you can still hear it. Have recently had fan and air pressure switch changed to sort out other problems. Would this be the fan again or something else? Could it be the pump or actuator and the sound is passing through to sound like its coming from boiler? Heat is getting round system fine.