Send your phone to Apple, they replaced mine (Europe) cause this is a massive quality control issue, if you check the forums it has been reported previously. I even checked the store models, all had the same problem, from normal Iphone 15 to the Max Pro.
Test your phone with this song https://youtu.be/G5xSLbYMr-I?si=r0U_wDIhrG39ZlIi 1st 10 seconds it will rattle like crazy.