Engine whine varies with speed

98 Windstar has developed a whine at speeds of 35 mph to 45 mph. There are exact speeds where the noise begins and ends, and it never varies. The whine changes in pitch depending on if I am accelerating or decelerating through those speeds.

I have read the other posts, and it seems that no one else has mentioned this specific problem. I thought it could be the idler pulley, but the noise doesn't vary with the rpm, just with the speedometer.

Also, when I first noticed the noise, I was sitting in the very back seat while someone else drove. That makes me think the noise isn't necessarily coming from under the hood. ???
Any help is appreciated.

Sounds like you may have a wheel bearing going. To confirm go to a road with no other traffic, drive at the speed where the noise is louded ,
and then slightly swerve to the left to see if the noise changes . Then try it to the right to see if it's different. Or just listen through some normal curves in roads . Whatever way you do it, Be careful of other cars and of course keep the van under control. At early stages of bearing failure it may be hard to determine which wheel. With the rear ones you can lift the van and spin the wheels by hand to see if one sounds rougher than the other. Using a stethascope can be helpfull.
Also check for play in the rear bearings by grabbing the wheel at the 12 and 6 o'clock and then at 9 and 3 o'clock positions and push and pull.
there should be very slight to no play. Compare both wheels with each other.
Good Luck

Thanks. I'll try that and let you know. I have also noticed a clunk or a knock under the van upon moving from a complete stop....don't know if that's related or not.

Thanks. I'll try that and let you know. I have also noticed a clunk or a knock under the van upon moving from a complete stop....don't know if that's related or not.
I also would not rule out the transmission.