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.

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.

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.