Strange noise on 1999 4Runner 4WD
 

I have a 4WD 4Runner with 170K miles. Ever since about 40K miles, it's been making a high-pitched noise, and I'm interested in finding if anyone else has experienced the same problem. Here are some details about the noise:
* The noise seems to come from beneath the truck, possibly the transaxle area, or from the right front or rear wheel. It's difficult to localize. It sounds like something making noise as it rotates, a loose grinding type of noise.
* The noise can only be heard when the car is moving at about 30 MPH or faster.
* The noise doesn't come on all the time. Sometimes, the car can go for days or weeks without the noise, and sometimes, the noise comes up right away. The weird thing is, sometimes, the noise would not be heard for a while. Then I would stop at a red light, and when the light turns green, the noise comes up when I start driving! There doesn't seem to be any rhyme or reason.
* The noise can be soft, like a bird chirping, or it can be howling very loud. Most times, there is actually a combination of those two variations.
* External temperature or humidity doesn't seem to be a factor. The noise can come and go whenever it feels.
* The noise tends to decrease or disappear when someone is in the passenger seat.

I had already replaced the rear bearings. If you have run into the same phenomenon, please let me know if you have found out what the problem was, and how it was fixed. Thanks!