Originally Posted by OTRtech
Probably the serpentine belt tensioner is frozen up and not keeping the belt tight.
There's always one thing I forget to mention...
I just put a new tensioner, serp belt, and idler pulley on it about a month or so ago.
I checked the fluids (they never change in level/run dry and are where they should be, so that likely isn't the problem)
Doubt it's the serp belt or its related parts since they are new.
Also, when the truck is not moving, it does not make this noise. I can let it idle all day long and I won't hear it. Once it's rolling faster than say, 45 mph, the noise really becomes prominent.