I think the best way to describe this noise is...when you have low power steering fluid and turn the steering wheel make that eh eh sound? Its kinda like that I hear it as soon as I start the truck 4.0L. My P/S fluid id full and it doesn't make that sound while turning. To me it seems to be coming from D/S of the engine.
I am not sure if it's an idler wheel or alt bearing going bad.