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After all that it was nothing to do with the boiler!!! It was a thermos flask sat on the worktop below. As soon as I picked it up to move it to allow access to the heating flow and return pipes, the noise stopped. I haven't worked out how it was making a noise, but it has now gone. So, happy days!!

I have turned off main stop cock and it has made no difference to the noise.

Thank you Boilerman2. Have now tried that but it has not made any difference. It doesn't sound like a water escape if that's what you mean. It still sounds like a leaking seal on a pressure cooker but there isn't a heat source to create the pressure.

I have a 7/8 year old Alpha CB24 combi boiler serving a self contained office attached to the house. Last night the boiler started whistling. It is still doing so. The CH is switched off. Drawing HW does not change the noise which sounds like pressurised air escaping. The pressure in...