Odd/Extra Revving Noise

My Rendezvous makes an odd/extra revving noise when speeding up, but it is not related to the increased RPMs. It does not make the noise when parked and revved, but will when coasting downhill and increasing in speed naturally, as well as when driving around town. The sound reminds me of a wind-up car. The faster the vehicle, the more "wound-up" it sounds. Any ideas?