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.
It sure sounds like something in the accessory drive. Once you are sure the belt(s) and pulleys are OK the next suspect would be a bearing. Water pump, alternator, tensioner come to mind. Ask your mechanic if he can use a stethoscope to pin point the noise.
Since we don’t know what engine you have, I’m gonna assume it’s the three liter. If it is, chances are it’s the cam sensor synchronizer. The cam sensor is mounted on the top of it. Remove the drive belt to make sure that’s not the sound. If you still have the noise with the belt off, see if it sounds like it’s coming from the back of the engine. If so, chances are the synchro will need replaced. It’s mounted on the engine behind the intake manifold. If you’re gonna DIY this make sure you follow directions and do the job properly.