Knocking pipes even when system off?

24 Nov 2008
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United Kingdom
I am a bit confused with my CH system at the moment as it is making odd noises. A few days ago we turned the CH on and I heard some whooshing sounds around the pipes - I assumed this must be air in the pipes although I have never heard it before.
The whooshing never returned but now the pipes in various parts of the house are making intermittent knocking/banging noises. Yesterday there was a banging around the boiler area itself but today I got home and there is a knocking sound in the bedroom/dining room (one room is over the other). The strange thing is the system was off and had been for 7 hours. I have the CH on now and it's still making a knocking noise every couple of minutes.
We had some mice in the house last year so I considered whether we had rodents hanging around the pipework somehow however there is no scratching and it is definitely a metallic knocking travelling in various parts of the pipe work.
Can air in pipes make a noise when the system has been off for hours? Does this even sound like air in pipes or something else?
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