My understanding is that as the steering wheel is turned the power steering pump cuts in until the wheel has no 'twist' or torque applied to it. So when the wheels point in the desired direction the pump stops.

On full lock the pump will stop but if the wheel is wound hard onto the lock the pump cuts in to try and drive the rack further - so the sound everyone should hear is the pump operating normally. On tight turns the tendency is to wind the wheel as far as it will go so as to maintain full lock - so the pump operates continuously but can't move the rack - thus loading the pump giving the 'more stressed sound' - all normal.

BUT doing this too often or for too long will overheat the pump and shorten it's life, when some of the other sounds will be heard. Squealing can be the drivebelt slipping under load (which may be better than heating up the pump).