IndyIan, thanks so much for your response and procedure notes to figure this out. I will do this tomorrow. I am hoping it is tires. I only have and just turned to 50,000 miles on this vehicle.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.
The tires were rotated back in May when I had my oil changed. (I never knew about the rears rotating backwards when they rotate my tires!)