99' Taurus Noise

Just recently my 99' Ford Taurus has been doing something strange. When I start it and idle everything is fine. When I start to drive everything is fine. When I hit about 30-35 MPH it starts to make a loud noise, kind of like the sound of the engine revving, but there is no difference in the RPMs. The noise goes away at about 40 MPH and comes back louder at 45 MPH. At about 50 it disappears again, and comes back softer at about 60 MPH. I have yet to drive it faster since it started making the noise so I don't know if the progression continues, but I was wondering if anyone has a clue what this could be.


Thanks,

Alex Star