I have an '08 Ranger, automatic. For the last several months, the truck has been making an intermittent high-pitched squeal/whine, but only after it has been warmed up (10–15 minutes driving) and only while it is idling. As soon as I’m driving, the whine stops; whine will occur both in drive (but stopped) and in park. The whine will not happen every day I drove, nor will it happen at every stop. When it does squeal, the squeal will fade in and out in volume.
An interesting kicker I’ve found recently: it seems to be dependent on electrical draw. For example, if I’m sitting idle with the lights on, and the truck is squealing, and then I turn the lights off, it suddenly squeals less. Still squeals, but less than with the lights on.
I have had mechanics check all the usual suspects, belts, etc, but have not found anything amiss. Nor can I actually get the squeal to occur when I visit the mechanics.