I think I've finally found and fixed the source of creaking or rattling from the tailgate area.
It occurs mostly at slow speed, i.e. when pulling or backing out of a parking space or slowly traversing bumps.
To confirm it was the tailgate I tried driving with the tailgate open and the noise stopped. Then closed it and it started again in the same parking lot.
About half way up the tailgate on each side there's a round bumper that I was unable to turn to adjust (maybe the plastic pulls off to expose a locking nut?).
I put a half inch ball of clay on each of these bumpers and closed the tailgate. I then could see that the bumpers probably weren't contacting the body when closed by almost 1/4 Inch.
I applied thick self stick felt to both the bumper and the body where the bumper should strike it.
I think the noise is gone.

 

My 2024 Disco had this exact issue from day one with terrible creaking from the rear hatch, and visible vibration of the hatch when closing rear doors. Clearly, the tailgate wasn't under proper tension. As stated by OP, this was caused by rubber bumpers not protruding far enough. The bumpers must make contact with the car in order to put the hatch under tension and prevent rattling.

As for the fix, YMMV, but in my car the rubber bumpers are adjustable. There's the bumper part itself, and there's a corkscrew part underneath. The bumper essentially "screws into" the hole in the hatch, and you can adjust it by (un)screwing it. I figured that that out by looking at all the bumpers, and noticing that some of them were sticking out more than others. One of the bumpers was screwed in too far, and sure enough, it was the side of the hatch that kept shaking. It can be a bit hard to get it going, but with some effort I was able to "unscrew" that one bumper far enough for it to start making contact with the car, and that fixed the issue permanently.

 

Thanks for the confirmation about those bumpers. Before I break one, did you remove the plastic/rubber cover to access the threaded part or just turn the entire thing?

 

My bumpers are a single piece of rubber: half bumper, half thread. I turned the whole thing. If you start from a fully screwed-in position, it can be hard to get it going, I had to use gloves on mine.

I unscrewed it and took a few pictures so hopefully you can see if yours look similar. There's the bumper itself, and the hole in which it goes. When I screw it in, I can easily tell the offset by counting the number of threads/steps outside the hole. In my case, the bumper on one side was fully screwed in (no threads visible outside the hole) and the other was two steps out.


Bumper alone.

Bumper screwed less than half way into the hole.

The hole.

 

That's great, thank you. That picture really helps. I was expecting a machine screw thread not that giant corkscrew which is what you did call it.
I was unable to turn mine before but I will try again. I was afraid I'd break the rubber piece off.