My 03 Ranger has done this ever since I bought it from a guy a few years ago. I never really questioned it but when I have asked around at different Ford dealers I have gotten many different answers. Basically, if I accelerate fast the noise does not happen till a stable speed. Once there its pitch becomes a higher pitch the faster I go, and a lower pitch the slower I go. Now if I accelerate slowly, It is there from what "seems" to be 2nd gear and up. Any ideas would be very appreciated! Thanks.