My 2023 1.5tsi SE L has a rear knocking noise from the suspension, it sounds like the boot isn't secure but it is.

Have you check the trunk left&right stoppers?

Known issues on the hatch model:

https://www.drive2.ru/l/710781491872801294/

I have this thing and it drives me nuts. I can't reproduce it intentionally, so every time I go to a workshop, they don't hear anything and thus don't fix anything. It is not a boot lid (it was fixed on my car), not a rear seats latch (fixed myself), but otherwise I have no idea what it is and it feels like coming from suspension indeed.

This started on my car a few days ago, and it really is annoying. I found this thread on the forum where it's discussed extensively:

Link to forum

The first thing mentioned was to spray the spring loaded dampers on the boot lid with silicone spray, and that seems to have fixed it for me, for now. Who knows whether it will last.

Apparently there was a recall on these parts, so I don't know if mine were previously replaced, but the comments were that on the new part the sprung loaded part seemed to move much more freely than the part it was replacing. So perhaps they start to seize up over time.

Thanks, interesting wonder if mine needs the upgrade, the car is great, but this is my 5th Octavia, & the first to do this

20 hours ago, DJD66 said:

Thanks, interesting wonder if mine needs the upgrade, the car is great, but this is my 5th Octavia, & the first to do this

It's been several days since I used the silicone spray on the dampers, and it's been completely fixed. So might be worth trying that first before ordering a new set of dampers.