When the heating is set to come on early in the morning it makes a whistleing sound for about half an hour. It sounds like it is self bleeding. The problem is it only seems to do it when there is a frost and I can only link this is due to the water being particularily cold.

If the heating is turned on to remove the chill on a summers evening this does not happen.

John Taylor, September 2007

Its not anything immediately obvious and you have not mentioned the model of your boiler.

I would suggest you could check the AAV is open and working and that the pump is bled of air at the central screw.

It is also possible the fan bearings are dry and making the noise. It would help to try to see exactly where the noise comes from

Tony

Tony Glazier, October 2007