If it's louder while turning it's usually a wheel bearing. Just put it on stands and remove the tires, and spin the hub on both sides. One may be obvious, but if you feel both you will probably feel a less than smooth bearing. I learned my lesson recently that these bearings can be the opposite side than what you would expect, or they have an inner and outer bearing. The front right for instance can howl in a left or right turn depending on which part of it is toast...
If it's as loud as you are describing I wouldn't want to travel at highway speeds on it. Try to see if it's obvious, and pay someone to replace it if you can feel it, or roll the dice.
