Just an update. My DIY of covering the whole lower tailgate with insulation did not eliminate the rattle noise. I finally gave up and brought the X5 to the stealership. My SA tried to remove all plastic covers around the tailgate and applied silicon to reduce rattle between plastic parts. I just got the X5 yesterday and did my own road test. My initial findings is that rattle noise has been reduced but still I can hear a few rattles once in a while in some rare uneven roads. Might need to bring back the X5 next week and ask them to put some foam insulation around the plastic parts in the tailgate section both upper and lower parts....