Power steering whine?

Why does power steering whine? I've just replaced pumps in the past, but now I'm wondering if this is the sort of thing I can rebuild myself. 

The gear/rotor thingys wear out and get galled.  Same reason auto transmission pumps whine when they wear out.

I've never rebuilt one but I did take one apart.  Look simple enough.

Air in the system can make them whine as well.  On some cars, they can have a small leak on the low pressure side that will draw in air.

The one on my WRX has been whining for like 50k miles now, even after pump and lines and belt replacement. I've learned I can drown it out with turbo whine :)

Looks like it was just extra air in the Mustang system. I read around and someone suggested hard lock to hard lock 20 times and now it's quiet. Fixed it! 

If the whining returns after some hard driving, then swap out the fluid.  Toasted fluid can sometimes cause whine when things get hot.  And worn out fluid is usually easier to aerate under hard use too.  And if that doesn't help, consider adding a cooler to the return line (hot, thin fluid can cause issues too, or hard use can just toast the fluid quickly if it gets too hot). 

In reply to mazdeuce - Seth :

Good tip. I had assumed that all Fords have noisy power steering pumps. Maybe just all the ones I have owned. 

The pump on my Olds Intrigue was hard in its fluid and would cook it and start whining. A flush and bleed would quiet things down. In retrospect I should have done that more often. 

My S197 has whined just a little when hard turning into a parking space from new. Seems to be a thing with the first few years. Got a little louder with the 275 tires, but never really changed otherwise.

Our Sienna's pump whines until warmed up. 

My plan is to replace it with the rest of the van still attached. 

rslifkin said:

If the whining returns after some hard driving, then swap out the fluid.  Toasted fluid can sometimes cause whine when things get hot.  And worn out fluid is usually easier to aerate under hard use too.  And if that doesn't help, consider adding a cooler to the return line (hot, thin fluid can cause issues too, or hard use can just toast the fluid quickly if it gets too hot). 

Thanks, and I think a second fluid swap is in the cards for this. Cycling the fluid has turned it about the same color as the fluid I just drained. Something icky lurks inside the system but I think it can be solved. 

In the past if i couldn't get something to stop whining i'd add some Lucas power steering fluid to it and that would fix it. Now i just skip the middleman and put some in anytime i have to refill a system.