I have BMW OEM crossbars with Thule's pull-top snowboard carrier(91726) attached.
Wind noises are noticeable from about 70mph.

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2008 X5 4.8i Black Sapphire/Black perforated int./Dark Bamboo wood trim, [w/ all factory options except rear DVD system]
+ aerodynamic kit/stainless pedals/illuminated door strip/3M Color-Stable tint/LED license plate light/Thule's 91726 snowboard carrier.
2006 X5 4.4i, Titanium Silver/Black int./Anthricite Maple(traded)