hissing power steering could be air in the system.
If it's the old style with the reservoir attached to the pump and not on top of the engine, they can be a pain to get all the air bled out.
When I replaced mine, I filled the reservoir completely to the top, rather than just to the "full" mark. Until I did that, nothing I did got the air bled from the system.
It does make a bit of a mess as you turn the wheels back and forth, because when the air bubbles come out of the system they can cause the reservoir to overflow onto the ground (I left my cap off during this bleeding procedure to keep fluid from going into the vent lines)
I had to fill to overflow twice before the air got out of the system and everything quieted down.
Other option is as Mogman stated above, the cadillac valve making the noise.
If something is wrong with the system, the fan can stay engaged 100% of the time.
The fan clutch is operated by the power steering system, so it would change sounds when hitting the stops at max turn.
Testing is simple. Open the hood and with the engine a bit above idle (1500 rpms) disconnect the electrical plug to the cadillac valve.
If it's working properly, the fan will engage and move a lot of air. If it's not working right, then nothing will change.
