Unsure whether the rattle is sourced from guide supports and at low speeds, if so Service Bulletin. Hopefully, rattle free by now.
http://www.allworldauto.com/tsbs/NHT...in_421770.html
NHTSA Item Number: 10056094
Service Bulletin #: SIB-33-03-14
Replacement #:
Vehicle/Equipment Make: BMW
Vehicle/Eqipment Model: X5
Model Year: 2014
Mfg Component Code: 022400 SUSPENSION:REAR:SHOCK ABSORBER
Date of Bulletin: 2014-06-01
Date Added: 2014-09-02
Summary: BMW: DUE TO GUIDE SUPPORTS BEING INCORRECT, THE REAR SHOCK ABSORBERS WERE ASSEMBLED, WHICH WITHIN A LOW MILEAGE CAUSES A RATTLING NOISE. MODEL F15 (X5). *PE