If it's more of a screeching noise as opposed to a creaking noise, I would have to say it's your power steering pump, pulley, or drive belt. Maybe your serpentine belt has stretched to the point the tensioner cannot keep proper tension. The quickest diagnostic procedure would be to check the belt, then the pulley for any play. If both are okay, check the Power Steering fluid level.
Mine started screeching when I got low on fluid after developing a small leak.