My month old 2019 X3 has the same sunroof rattle. And it seems the rattle/noise is related with humidity of out side sunroof rubber seal. This is because if I do a car wash, the noise is gone for the day but it comes back next day once all humidity is dried out.
My sunroof rattle started 2 days after I picked up a new car from my dealer. And I spent a lot of time to pin point the area of the noise. It seems the noise comes from rear passenger side fixed glass of panoramic sunroof. If I apply some pressure to the center rear glass, the noise is a little diminished or it makes slightly different noise.
Since I found where and what rattles, I visited dealer and explained everything what I and this forum members found. On the first day, they called me they were unable to reproduce the noise/rattle. Next day, I visited the dealer again and I drove with one of their engineer. The engineer identified the noise, so I left my car again. At the end of the day, dealer called me and they refused the fix. They said they tried 4 of their 2019 X3 loaners and ALL OF THEM have the same noise. So it is "characteristics" of 2019 X3 and BMW won't authorize the fix unless they have some service bulletin. I got completely speechless for a while.
I also showed them the screenshot of work order bmtespar posted. And told them another BMW dealer fixed it already. The service adviser told me, they can't identify the work order since it doesn't have model year or VIN number.
I complained again why you can't fix it since you know where and what rattles and the service adviser keeps repeating, "I understand.. I'm on your side.. it's unfortunate but it is a characteristic of the car".
After unhappy experience from my BMW dealer, I ordered a rubber protection product and applied it to outside of rubber seal as forum members here recommended. Now the noise is gone at least for two days until today. I hope this lubrication lasts long.