I have a Potterton Performa 30HE about 3 years old. Approx 2 months ago the boiler started emitting a 'squealing' noise usually after the heating had been on for about 30mins, it would continue for 5 - 10 mins then stop. This noise will 're-appear' at irregular intervals thereafter. Once or twice the noise has appeared when filling the sink with hot water. Any suggestions gratefully recieved.