My truck has this harmonic howl also, between 60-70mph is when it gets really bad. I started noticing it this spring and back then it was very faint and almost unnoticeable. It also did the shutter from take off every now and then. I did a fluid change and added a little bit of friction modifier and have not felt the shutter since, but the howl has slowly become more and more noticeable to the point where passengers ask what the noise is? I can hear it follow the drive train speed, but depending on where the throttle is at the noise will increase and decrease. If I match the throttle to the drive train speed while going down a hill (take the pressure off the drive train) the noise completely goes away? I did U-joints on the rear drive shaft 12,000mi ago hoping those would solve it but no go? I am at a loss