Brilliant bit of lateral thinking, and a useful one to have on record for anyone else who finds themselves chasing a mysterious rattling noise at an unsociable hour.

The instinct to fear the worst (compressor, hidden pipework, walls coming down) is entirely understandable. The reality being a trapped air bubble dispatched with a momentary flick of an isolation valve is a satisfying outcome by any measure. Well done. Let's hope it stays away.

So that phenomenon is usually referred to as "water hammer", although that's a general term which covers several potential faults.

Unless there's opportunity for air to enter the system again, it's usually a one-off air bubble, created from air that was dissolved in the water when the system was first filled. The bubble gradually grows until it's large enough to be propelled somewhere significant. Therefore it shouldn't re-occur once it's been urged to find an air-release valve.

Water hammer is worse when the pipework has sharp right-angled elbows. So that's another reason they should be avoided.

Thank goodness this happened to a 'senior' Member here. It would've caused significant worry to most heat-pump users, and may have disappeared again once an installer arrived to diagnose what's wrong.

I also have a noise in the night. It is a loud cracking sound occurring at or shortly after the temp rises to 50c or above. The attached file is not my tank but the sound and tank is the same as mine. Since I am now running at 45c  and no legionnaires cycle I do not get it. Octopus sent an engineer but he could not reproduce it so I waiting anther engineer so I can show him the issue. Unfortunately the attached file though only 5 seconds is too big. I'll have to reduce it.

This may work hopefully now

Posted by: @jancold

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I also have a noise in the night. It is a loud cracking sound occurring at or shortly after the temp rises to 50c or above.

Sounds like something expanding somewhere, perhaps then hitting up against something.

This is my setup which has more components.

Yes it does sound like that @jamespa  not really like water hammer. I wonder if there could be a problem with the expansion tank as it only occurs at higher temperatures. But some Octopus engineer must be aware of this as the clip appeared on FB. There is no water in the tundish indicating that the pressure valve did not operate. Annoying as mostly the installation works well.

Posted by: @jancold

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Yes it does sound like that @jamespa  not really like water hammer. I wonder if there could be a problem with the expansion tank as it only occurs at higher temperatures. But some Octopus engineer must be aware of this as the clip appeared on FB. There is no water in the tundish indicating that the pressure valve did not operate. Annoying as mostly the installation works well.

FWIW I used to get expansion noises from pipework under the floor with my gas system.  Inevitably pipes move a bit, the coefficient of expansion of copper is quite high, and if this causes them to come into contact with something it will make a noise.  Yours may be no more than that and if so its nothing to worry about.  That said if the video you posted is the sound you are hearing it does sound like a more distinct noise.

yes, if you are standing by it you jump it is very loud, not what you want to hear at 0130!

I can't really suggest anything other than trying to narrow down the source location.  Sorry.

I appreciate your help and when I find the cause I'll put it up here in case others have this.