What is wrong with my Potterton Kingfisher 2?

My Potterton Kingfisher 2 floor standing gas-fired Central Heating boiler has started to make a very odd noise once it has been running long enough for the system to be hot. The noise is quite loud and sounds like an electric kettle 'singing' intermittently just after it has been switched on. It only occurs when the boiler is 'lit' and it can be heard throughout the house via the radiators. So far I have changed the circulation pump and run de-scaler through the system for a week. I have also reduced the gas pressure setting one level. There is no evidence of any water leakage and the radiators get normally hot. What is it trying to tell me?