Re: very light humming when cruising
If you keep up with transmission services a difference of oil shouldn't create an abnormal noise. Synthetics allow better wear characteristics and flow better, but conventional ATF would easily work in your '97 Ex. Ford uses Mercon V, which is a full synthetic fluid and I don't think you can even buy previous Mercon brands anymore.
If you haven't already, check the fluid level to make sure it isn't overfilled or underfilled. I've had some underfilled transmission make some noise, but not constant humming.
Some things to look at are properly wearing tires (I know, you think you'd hear them hum all the time, but you'd be surprised). Next, check out the differentials. A worn differential, such as ring/pinion gears can cause a hum if they're not exactly aligned.
On my Dodge Durango, it always emits a light humming noise when I give it throttle. It's due to mostly to the injectors and such.
Lastly, just as an experiment, try driving the vehicle in 2nd and 3rd gears up to 40mph. RPMS will be high, but it'll keep over drive locked off. If the noise goes away when your in a lower gear then I'd have someone look deeper into the transmission. If the noise remains when in the lower gears, then I wouldn't worry about it - your baby's gettin' old!
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!