You can flush out the system. Here's how:
1. Disconnect ignition or make it so you can crank the engine and not have it fire.
2. Take the low pressure line (the one that's held onto the pump with a hose clamp) off and put it so it will drain into a bucket.
3. With the help of someone else, crank the motor over so the pump starts to push out all of the old fluid, while pouring in new high quality power steering fluid. Don't let the pump run dry!
4. Turn the steering end-to-end once or twice, but don't hold it there.
5. Stop once the fluid runs clear and looks new. Re-hook up everything and try it out.
I did this in my taurus after I got tired of listening to the pump all the time. I used to use just automatic trans fluid in the power steering, but after putting in high quality power steering fluid and flushing out the system, it's been pretty quiet.