Watch the engine RPMs as you turn the wheel, and watch them dip as you turn the wheels all the way to either direction. This is the power steering pump suddenly having a large load to deal with because of how power steering works. The wheels reach the end of their turning travel, but the system keeps trying to turn them, causing the extra "load."
Every car with this type of power steering does this; some pumps just make a really loud noise once they have this huge, unchanging load to pump through, If adding\changing fluid does not help, either take it to a mechanic and have him check the belt tensioner to see if its the belt squealing on the pump's pulley due to the extra resistance, or have the pump replaced\repaired.
I recommend you don't turn the wheels *all the way* for long periods of time as the system was not designed to do this. (hence the loud noise) (EX: when you have to turn ALL THE WAY, do so, and turn back maybe, 10-20 degrees to ease the load on the power steering system and proceed with your turn as if you werenít able to turn that far in the first place. Thatís what I do.)