Banging noise (air?)

[Grenage]

Very occasionally, maybe once a day when the heating is on, there's a banging noise like someone knocking loudly for a second. The only thing that's changed is that we reinstated a radiator after about a year of not getting round to it.

I'm assuming that there's some air in the pipes and it makes a noise when it gets to the pump or boiler. Does that sound likely, and if so what's my best course of action?

It's an old open vented system.

 

[Madrab]

That would be wonderful but I'm not sure that the company would be prepared to absorb the travelling cost from Ontario to the UK, either that or the quote is going to be rather high.

PS .... this is a UK based forum and the OP is in Hampshire, which if I remember is in the southern part of the UK ....

 

[Grenage]

I've bled the radiators and the pump; it only seems to happen when the pump is engaged for CH, and only for a second.

DHW doesn't cause a problem.