I have a 2021 X3 that has a rattle/buzz near or inside of the ceiling/headliner, somewhere around the right rear passenger seat's ceiling handle. The driver side seems fine, i.e. no sounds. But the rear passenger side has a maddeningly subtle buzz/hum between 20-60mph. I've run my fingers all around the inside by gently pulling the headliner down away from the sliding panel's metal track, as well all around the outside, but I can't pinpoint the problem area or find anything loose/ajar. When I apply some upward pressure to the area, i.e. push up on the ceiling toward the sunroof, it seems to stop, but I still can't pinpoint the exact spot or figure out what might be loose.
It's probably loose screws that are supposed to hold the metal frame of the sunroof. See this link for more information.
I had the exact same problem with a Toyota.
The reality is that there is undoubtedly something not affixed properly or something accidentally left behind during manufacturing.
If it's the former, it could be a headliner attachment, a wiring harness, a wire, a mounting plate, a mounting screw, or a piece of moulding. If it's the latter, it could literally be anything from a piece of plastic (most likely) to a piece of metal or glass.
The fact that it only occurs on one side is the giveaway.
The only way to truly fix it is going to be to disassemble the area and see what is causing the noise.
As an alternative, you can try buying new tires. Every tire has a different subtle effect on vibration, and it's possible new tires will make the sounds go away.
Oh, and if you were wondering about that Toyota with the same issue, a piece of the sunroof mechanism was too loose and was causing the noise. I disassembled the sunroof and reinstalled it. That fixed the issue.