Our pressurised central heating boiler is making rattling/banging
noises, but only when the hot water is on but not the radiators. It
sounds like air in the system; all the radiators are bled.

So I wondered if there was a way to bleed any air from the hot water
tank coil. I can't see any way, but there are two air vent valves on
the inlet and outlet. They look a bit like this...

https://www.jtmplumbing.co.uk/heatin...BEgKf4_ D_BwE

....or even...

https://goo.gl/2ctbUC

On ours, the plastic cap/button is screwed down tight. I think it
should be loose. Should it?

Cheers
--
Clive