Anyone who played Skyrim will know what i am talking about. It's a quite low, worm sound you hear when you are in the open. It gives the information that there is wind and it sets the atmosphere. Also if you sprint there should be a certain ammount of wind noise and you should not be able to hear as good as somone who is standing still. Same goes when you are close to a helicopter. This should be implemented very carefull not to be anoying but enrich the experience.