I've been experiencing this strange noise for well over 2 yrs on my 01' PreRunner. I realize that is long time to wait to find an answer, but here I am. Actually, I first remember it a few months after I got my truck back from the dealer due to an accident. The problem was and still is that the noise does not happen all the time. I had one mechanic look at it, but during the time he had it the truck never made the noise. It seems to be happening more frequently now. The best I can describe it is it sounds like a high squeal (almost like bad bearings or something is loose). It can get rather loud and its annoying as all get out. It sounds like it is coming from under the back seats. I have the Extended Cab. The key to this noise is the pitch of the squeal elevates when pressing on the accelerator. When I take my foot off the gas it most often goes away, and if not totally away it always decreases in volume and intensity. What would this be? Fuel pump? I have no idea? If it was the fuel pump you would think after 2 yrs the thing would have failed already. Any input would be appreciated.