Thanks for your replies.
It's happening when the boiler has heated up, i.e. about 5-10 mins after firing up for heating. And it's more of a higher pitched "whitle" than a droning fog horn.
I inherited the newly installed boiler when we bought the renovated house 18 months ago. I have a...
Can anyone advise the likely cause of this please? It only occurs in heating mode (not water) after the bolier has been running for a few minutes.
I wondered if it was a pump problem as it doesn't occur when heating water, but it doesn't sound mechanical.
The boiler is just out of...