I have had a weird hissing noise under power for a while which I thought was from my trans. Flushed it twice, checked for loose converter heat shields, replaced serpentine belt, nothing. Last week I tried flipping to 4High and the sound went away.
So I changed the t-case oil and now it's nice and quiet. I have not heard the sound since. Some times things are so obvious that I don't even see them.