I have an 04 2500HD w/allison and 184K miles. Recently, under acceleration and load (trailer) I've noticed a whining/scream sound coming from what I think is under the hood. This seems to only happen when goining though 1900 - 2100 rpms. There doesn't seem to be any oil pressure drop or voltage. Transmission temps, shifting patterns etc. seem to be OK. Truck seems to run normally otherwise. Someone suggested that power steering fluid might be low. Checked that, it was, added more fluid and the whine is still there.
Any thoughts

Has the belt been replaced?

It's not the turbo whistling, is it? A very faint whine as the turbo spools up.