ss wrote in :

On 07/05/2019 10:38, John Rumm wrote:

Sealed system or vented?

Are you sure that is not air being sucked in you can hear?

Sealed.
The sytem is pressurised and heating all radiators, however,
I increased the bar pressure and that appears to have sorted it.
The bar pressure is now half bar more than it normally is.

surprised the boiler didn't shut down due to low pressure.