Why does my Jeep hiss at me when I turn the steering wheel?

I noticed a rather unusual noise coming from (of all places) the steering column: hissing. I heard it the other day when I was in a parking garage and the surrounding area was rather quiet (nobody around, windows up, radio off, etc.). I only hear it when the steering wheel is in motion (rotating)... almost as if there's a mild vacuum leak. The steering pump doesn't make any unusual noises outside of the Jeep, so I'm at a loss. Nothing seems to be malfunctioning; I was just curious what that noise might be.

At full lock it's the bypass on the power steering pump...I wouldn't recommend doing that a lot. I'm not entirely sure of the cause for the rest of the turn; I have been lead to believe it's a normal hydraulic pump noise.

Well, I found out that I "misplaced" the power steering reservoir cap, haha. Didn't seem to change anything, though. It's one of those Jeep noises, I guess.

My XJ steering knuckle has often made noises like you describe. It turns into more of a whine as time goes by. I'm a noob so I could be wrong.