We have an Ideal Response 100 combi boiler. For a while now, after the central heating has been on five minutes (and the pressure gauge has gone up approx 0.2), the boiler whistles for about five minutes and then stops. It then whistles immediately after the ignition is in use for a short while . The whistling has now got a worse - louder, almost a squeaking sound. There are no cold spots in the radiators. Any ideas what is cause be? We will probably replace boiler in the spring/summer when engineers are quieter anyway, so just hoping it's nothing major about to go.