MosH,

according to the THULE site fitting guide, it appears THULE recommends the medium size fairing see here (Thule - Thule Racks - Rack Accessories) 871XT 38" for the X5..given your description that the 872XT 44" if you squeeze it between the factory rails pops up in the middle, I think your solution might be to go to the shorter length fairing and see if that does not cure the fitment issue.

this sounds like a great set up (which others may have as well)..I would be interested to hear how the fairing does on reducing the whistling noise.

DutchK