in certain conditions, air current will create noise as it goes around the racks...it's inevitable...the remedy is to take the racks off if the sound becomes annoying...especially if you are not using the racks for anything. Hopefully they gave you the keys to unlock the clamps so you can access the bolts that secure the racks to the rails.
To remove the racks just lift the rubber cover and stick the key into the lock to free the clamp that covers the bolts that hold the racks to the rail...then take the 10mm wrench from the tailgate's toolkit and loosen the bolts. This allows the racks to be slid back to the opening in the rail and taken off easily (if you have the keys).