So it is very odd...I know I am of no help in the description. It is more of a high pitched whistle, it does increase with increases in RPM and road speed. At 1st I really thought it was my tires on a smooth newly paved road. But when I let off of the accelerator the pitch decreases. So I thought today ,...maybe yes perhaps it is my tires. It would make sense if I let off of the gas pedal the tire rotation/speed would decrease and so would the noise. However, so would the RPM of the engine as well. I am going to drive tomorrow on an old road going to work and see if I hear the noise. I will also test it again when I go to work in the morning at idle in my garage and will increase the RPM.
I know what the whine of my power steering was causing, when you turn your wheels in either direction. That I would classify as a whine. This is not that noise. And that noise stopped once my mechanic changed the power steering fluid and put in a new O'ring, thankfully. I have watched videos of a whine noise, growl almost of an alternator, and no I don't have that noise either, thank God!
My blizzak winter tires actually had a slight acceleration whistle/whine when they were new on my Focus, increased pitch with road speed and went away with letting off the throttle.
Did you get your tires rotated at the mechanic? Maybe one of the rears needs a few miles to adapt rotating "backwards" on the front now?
In a garage or parking garage, or even a quiet spot beside a brick wall, with your window down you can rev the engine in N to see if the whine is rpm related. In the same spot, put it in D and hold down the brake and gently press the throttle and see if you get vacuum leak whistle. Also in N, hold the rpms at 2000, and turn off all the electrical loads and then turn them all on, high beams, rear defroster, seat heaters, and turn the steering wheel slowly, all which should load the alternator right up and perhaps it likes to whistle while it works?
If the noise is not there then, then its probably transmission or tires. My Focus has a bit of transmission gear whistle too, but its not too noticeable, unless I have the fan speed on 1 with a perfect smooth quiet road.