Making a noise?

I have a noise coming from my loft (I think as I cannot pin this down). It sounds like a kettle boiling.

It lasts for approx 15-20 seconds then stops. It starts again after approx 15 minutes and has been doing it for approx 2 days now. Happens every 15 minutes, even though the central heating and water is OFF. I have a combi boiler behing my fire downstairs and a header tank (i think) in the loft. I can only hear the noise upstairs so I dont think it is coming from my boiler ??!!

Anyone with similar problems or know what it could be?

I have read on other posts that maybe the radiators need bleeding as there is air in the system??

Others have said that I need a new pump or fan??

Any advice welcome.

Thanks in advance