I also have a 1997 Explorer XLT V8 5.0 2WD (116k miles) with a noise similar to what you described. In my case, it started a couple months ago as a metallic chirping and I immediately thought it was a bad bearing somewhere. I couldn't pinpoint the sound 100% but using sound triangulation with a friend we both felt it was in the area of the alternator. So, I replaced the alternator but it made no difference.
The rhythm of the sound matches the rpm of the engine. A/C on or off has no apparent effect. The sound has gotten worse and now sometimes sounds like bearing growl similar to when power steering pump fluid is low. However, it's not as loud as a dry PS pump and the volume decreases after driving for a little while but never goes away completely. Sometimes it just has the metallic chirping sound. I took it to a mechanic but he couldn't pinpoint the source with a scope either. He said it is definitely something on the belt because the noise goes away when running without the belt. However, he said all pulleys, power steering pump, water pump, compressor and alternator (which is new) turn smoothly. Initially, before looking at it, he thought it could be the cam shaft positioning sensor but the belt removal test eliminated that possibility. I'd really like to find a solution because I'm certain something is not right. I don't want to wait and find out the hard way by breaking down on a road trip. Anyone know of a cause and/or solution?
