Air in the power steering system. Air can enter the power steering system when a line is disconnected or damaged, or when the fluid level is too low. Air can cause the pump to cavitate, which can also produce a squealing noise.
Worn power steering pump. As a power steering pump wears out, it can start to make noise. A squealing noise is one of the most common signs of a worn power steering pump.
Damaged power steering belt. The power steering belt drives the power steering pump. If the belt is loose, damaged, or worn, it can cause the pump to squeal.