Jeff is probably right. If you don't have time to check it out properly take a bar of dry soap and rub it on the belt as much as you can then start it up and see if the sound leaves. Should that not quiet it down. Start checking your pulley
The noise could be just the pressure relief valve opening to bypass fluid back to the reservoir.
When you turn your steering wheel, a valve in the steering gear directs fluid under pressure to a cylinder to assist in steering. When the spindles reach their stops, the cylinder can't move any further, although the valve is still trying to send fluid to it. Since the fluid is not compressable, something has to give, and to protect the system, the manufacturer includes a relief valve that opens at higher pressures than are normally experienced during normal steering operation. Fluid flowing across this valve sometimes emits the squealing sound that you heard. In such a case, the relief valve is doing exactly what it was designed for, and should not cause you to worry. Of course, a slipping belt can also make a squealing sound, but it sounds like you've inspected that possibility and eliminated it.