If you only hear the noise at full steering lock, but the noise disappears as soon as you let the steering wheel turn back a bit, it's probably the power steering pump straining to recirculate steering fluid. At full steering lock a pressure relief valve activates to limit pressure on the output side of the pump. When the output is blocked, no fresh/cool fluid from the reservoir is drawn into the pump and the fluid only recirculates through the system, getting hot in a hurry and causing the pump to get noisier. That's why you should limit operation of the steering at full lock to brief periods (this is true for any car with power steering.) If you don't, the hot fluid could eventually cause the pump to overheat and fail.
