Spray a bit of WD40 on the belts and see if to goes away. Dont spray to much because it might cause the belts to slip. I usaly spray it on while the engine is running and stop spraying when the noise goes away. There is also a lub stick you can buy for belts, its like $2 from an auto store.

Although, i dont think that its your belts making the sound, as you say its only happening over 100km/h. Taking a guess it could be your gear box or something that is just making the noise because of the air rushing through.