Got to agree with Steve about the steering boxes but pumps fail and they'll scream bloody murder if they have air in them. Hydraulic fluid doesn't compress but air sure does. With the front end off the ground and the engine at idle run the steering from side to side and see if you can work out any air in the system. Hold the steering at the right and left stops for a couple seconds to allow any air to pass thru the box. Sometimes you have to do this 20 or 30 times or more. If you see bubbles in the reservoir there's your problem. Make sure the reservoir is kept full when you do. There's a complete procedure to do this in the -20 manual.
To check if it is the hydro boost:
While turning the wheel on either direction, step on the brakes. If the whine goes away or decreases... you can blame it. Air is getting in. If not the lines, might be its seals.
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