I found a couple similar posts, but without any replies...
I have a 98 'tour 2.5L V6, 52k miles. Just recently (in the last month or so) I've noticed a very high-pitched squeal/whistle coming from the engine, like the sound a TV makes when you first turn it on. It's not there when I start the engine, but only after about 15 minutes of driving. The pitch seems to go higher as RPMs increase.
I've had a few people listen; one suggested belts, one suggested pullies, and one was adamant that it was the alternator. I'm leaning towards the alternator myself, anyone have any other suggestions?