High Pitched Squeeling coming from a 93 Previa

[Stick_Shift_Previa]

I drive a 1993 Toyota Previa minivan with a manuel transmission (one of the few) and lately it has been making a high pitched squeel that is not related to the brakes.

One evening I began to hear this loud screach coming from what sounded like behind me. I orignally thought that I must have been dragging some piece of metal behind me, but I was not. The first thought was that my braks were in need of replacement but the screaching happened when I wasn't braking. After hearing the screaching I stopped and let the car sit for about 30-45 minutes before attempting to drive it again. After that I did not hear the noise anymore. Since then the noise has come back but to a much lesser extent. When I reverse there is a high pitched squeeling sound that seems to come from behind me. Every now and then (and it is happening more frequently) the noise will kick in while I am driving. I have not been able to determine a pattern of when the noise starts (how fast I'm going, how long I have been driving, what gear I am in, am I on a hill) but I am still concerned. I have since taken the Previa to Toyota for an inspection but they said that unless I could reproduce the noise that they could not help. Has anyone had a similar problem in their Previa?

If anyone could help me out or give me any advice on how to remedy the problem or tell me the extent to which my car needs repair then I would be very appreciatative. Thanks.

-Kristopher