Well.............of it rocks with battery out, my next suspicion(actually first) would be Magnets causing the rock. To thick or not pressed fully in place.
Nope, it's just sloppy milling on the base that's the real culprit.
See how irregular that edge is above the fire button and to the left? The battery door is milled straight, but doesn't have a flush edge on either side to work against, hence the slop.
I'm thinking they ran a production run (or two, or three...) with it like that, looked at it, said "Screw it, ship it" (or the Mandarin equivalent), and later batches (or earlier batches, whatever) didn't have that machining problem which is why some folks don't have the rattle.
My first fix was some rolled up Scotch tape (cello tape for my Commonwealth friends) on the inside of the case, along either edge, that worked very well, since it was sticky side out, really keeping the door in place but still easy enough to remove. It basically acted like little bumpers preventing the metal-on-metal contact, and the exposed adhesive increased friction making the contact less likely to start. The pain of that method was that I had to remove the tape tube running along the outside edge of the battery every time I wanted to swap, and at home chain vaping, I'm swapping every hour or two. Got to be annoying.
So, I layered up some electrical tape on the inside of the battery door instead. Took a lot more material, and it's a sloppy job, but it works, and it's no fuss at all removing the battery or the door now. Downside is, it isn't quite as
slop free, but it's totally
noise free, and it looks like raw asz when the cover is off:
Next step is some good black silicone sealant replacing the tape on the door, when I get motivated enough and can wait around long enough to let it dry before using the mod.

Or maybe try some Bluetack.