What could this noise be?

[Dutch Rutter]

I'd also be suspicious of pulleys, pumps, belts, and alternator, just by the pitch. Easy way to rule them out would be put the trans in neutral and give it a rev when the sound is happening. See if the sound speeds up with the motor or slows with the ground speed.

 

[thecantaloupeman]

How fast were you going does the noise slow down as the truck does? Also when its happening turn the truck off and see if it goes away.

That's a good idea. It definitely is wheel speed related, but I'll give that a try in the neighborhood.

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