like fingers on a blackboard

Hi - I have a 99 Windstar. Often when I turn my steering wheel there is a horrible squeaking/squealing noise coming from the left front wheel. It sounds like fingernails on a blackboard! I've recently had my brakes replaced so I don't think that's it. Any ideas?

Is this while the vehilce is moving?
If so.....one cheap and easy cause might be the metal sheild that is on the inside, bottom of the front rotor......covers the bottom of the rotor to protect it from road crud. If it is bent very slightly, it could rub the rotor slightly and make a nasty sound.

Does it happen ONLY while you are turning the wheel, and stops when you stop turning the wheel?
That could be a tie rod end grinding, from lack of lubrication.
If it is in the very early stage of this, you might be able to use a grease needle on the end of a grease gun to poke into the rubber boot over the tie rod end and pump a SMALL amount of grease in there.

Also check the level and condition of the power steering fluid.
I think your '99 will use Mercon rated Automatic Transmission Fluid (ATF).
Do NOT overfill. Look for bubbles in the fluid,, as they will make a LOT of noise.
The windstar is known for cooking the power steering fluid.
There are posts on this forum as to how to flush out the old fluid, and replace it with new fluid.

Look over the power steering lines to check for any damaged or pinched conditions.
Also, look for signs of leakage.

Does it make this sound with the A/C off?, as in the compressor not running?
You may have to select "VENT" or OFF on the selector.....mine will be on in all the Heat, A/C, and defrost selections.
If it only makes the sound with the A/C compressor on, then the problem may be more relative to the A/C compressor.