Ive been noticing more and more latley that while my trucks in 1st or maybe 2nd gear, say, slowly coasting around a parking lot for example, theres a whining noise coming from either the rearend or the tranny (cant tell which). It gets louder and more high pitched as I move faster then disapears as soon as the truck shifts gears..

Is this something serious like my transmission on its way out? From what I gather a lot of people seem to have a similar thing going on.. From what I can remember my truck has done this since I bought it, but latley (and maybe its just me) I seem to notice it more like maybe its getting louder.. Can anyone shed some light on this?