well if you have checked the tie rods, balljoints,shocks and struts what else could there be besides a rack ? you say it only does it when you turn so i think that elimates a hub, so you may want to check the powersteering fluid and the rack, which i really do not know how you do, just try to go by a process of elimation that is what always help me find my problem i even get called crazy for talking to myslef sometimes but it helps