I have the same problem when I m on 2WD.No problem at 4WD.I can hear a sound like the one described.I think it comes from somewhere on the passenger's side,but I m sure it looks like coming from transmission.It becomes louder when I change to longer gear,from example from 2nd to 3rd and 4th.It seems to do this when the engine doesnt have much torque(for example 3rd and 2000 rpm+pushing the throttle) ,I know that if I use longer gear at low RPM a noise like that sounds,but its much more intense.Something like "GZZZZZZZZ".
It sounds like gears rotating.I dont think its normal either.I thought that maybe due to body lift and the fact that the shifter's rubber boot is open a bit (I can see the road) ,the transmission noise seems to be louder,but I dont know....
The truck has its services more often than required.