Whirling noise

I have a whirling noise that I can't seem to find what it is. I heard it and immediately thought wheel bearing. However, I got my wheels rotated and balanced and the sound followed the wheel, so then I thought tires. It's been to two different shops, and I have been told the tires are fine (they have about a year of normal commute driving on them) so I am clueless as to what the noise is. It is speed dependent; at low speeds I can barely hear it. I hear it the most when deaccelerating on smooth roads. Any help would be greatly appreciated.