Question? Are you hearing this noise when the Steering wheel is turned either way, completely to it's stop? As in, have you turned your steering wheel as far as it will allow the wheels to go?
If the answer to that question is yes, then the noise you are hearing is you putting undue stress on the PS Pump, and asking it to do more than it was designed to do, for longer than it was supposed to. This will eventually lead to your PS pump's failure.
If the answer to that question is no, then you most likely have a faulty PS pump.