I have a baxi combi 80 e which is approx 7 years old I think. When the heating is on and the boiler temp control is past halfway it makes a really loud whistling noise. When the temp control is at half or less this stops. I had Brittish Gas out today as I have a homecare agreement and the engineer said that this is limescale in the heat exchanger and the only way to cure it is a power flush at approx £600 or to leave the temp control down at half. Is this correct?