I just purchased a 2004 F150 XLT Supercrew 4x4 with the 5.4L V8 and 3.55 LS gears. The truck has 111k miles on it and is in excellent condition. This was a local truck and I knew where the truck had been sold and serviced so I was able to confirm with the dealer's service department that the truck didn't have a history of major or recurring problems. This truck was bought to be used as a second vehicle to keep me from beating up my recently purchased Suburban LTZ whenever I need to haul something or go to the hunt club. I have already put around 400 miles on the truck since I got it last week and the only thing that I have noticed is a whine that seems to be coming from the driveline when I reach between 48-52 MPH. It isn't there up until around 48 and it goes away above 52. I just had new tires put on and it isn't the tires as it did it with the old worn out BFGs that were on there when I bought it. There aren't any vibrations or shimmys and everything else seems perfect. It rides almost as smooth as my Suburban that has the Autoride suspension. I had a local mechanic put the truck on a lift before I purchased it and check the front end components as well as the driveline and everything was tight and looked as it should. He even commented on how tight and clean everything was. Does anyone have any ideas about what it may be so I know where to start looking? Rearend, universal joints, etc. It doesn't seem to be affecting the way the truck drives for now but I don't want to overlook a more serious problem if I can avoid it now but I just need to know where to start.

Thanks,
SCScrew