tonyjeffs wrote:
We had the same problem immediately after new ch installed, but it
went away after a week. I figured it was bits of trapped air that
needed time to sort themselves out.
Same might happen for you.

If not, is it near the boiler, or in the radiator loops? You could try
isolating parts of the system by fitting isolator valves and see if
you can localise it.

You will get this on any system. There is always some outgassing from
the pipes. In practice the bubbles get carried to a local high point -
one rad always has a cold top. Bleed that, re-pressurise if needs be,
and robert is a relative.