Hmmm could be aliment do you have any unusual tire wear. Wheel bearings do make a kind of woshhhing noise when going strait and when you turn the wheal sometimes it will go away. My guess it is a wheel bearing. Jack up your car and see if you can move the wheel side to side like this http://www.youtube.com/watch?v=5khjK0y6PIw
most likely its a bad wheel bearing. It probably isn't aliment but my friend had a car that had a bad aliment and forever we thought it was a wheel bearing.