I read most of the other posts on this, but it seems like mine is opposite of what others are saying.
When the truck is a normal temp, I can be driving and hear it very faintly. When I go to stop a a light, it will become very loud before it goes completely away. I was told that it could be a sticky IAC valve. I cleaned it, but the problem didnt go away.
I am driving a 1998 Ranger 4x4. 4 door 4.0L V6 5 speed.
It's got 225,000 plus on it. Still going strong.

 

Any noise idling after cold start, or if you REV the engine cold?

If so with engine cold release tension on fan belt and remove it from crank pulley.
Start engine, you can run cold engine 2 or 3 minutes without water pump.
If noise is gone then you have an accessory bearing going out.
Spin each pulley to find out which one may be failing, including idler and tensoner pulleys


Another noise maker can be the fan clutch.

After engine sits over night or 5 to 6 hours, pop hood and try to spin the fan, it should be tight, you can move it but not spin it
Start engine and run it for about 1 minute, then turn engine off.
Now try to spin fan, should spin easily, some resistance but will spin