What causes this sound wo wo wo?

[COBALT]

What happens when you speed up and/or slow down in this RPM range?

What about going around corners (1500-2000 RPM isn't all that much)?

If you're sitting still, and your tranny is in N and you rev the engine do you hear the sound?

Find a long hill, and start at the top. Put your truck in N and coast down. When you reach that familiar RPM range do you hear the noise?

With that same hill start at the bottom, and drive up until you reach that RPM range (I assume you'll hear the sound). Does the noise sound worse than usual? Does it sound worse then when you coasted down hill?

Is the sound coming out of a particular side of the truck?-front, rear, driver's side, passenger's side? Have someone ride in the back on a flat straight road with no other cars and listen...

With what you're telling us it could be anything engine, trany, driveline, or differential related! Narrow it down.