i had the same issue a while ago and it drove me nuts - i checked all the obvious ones (tie rods, ball joints, bushings, sway bar links) but i turned out empty - i took it to a frame shop and i was turning the wheel back and forth and the mechanic was under the front end touching all the links and joints - after about 1/2 an hour he figured it out: it was the trackbar bolt! not sure if you have the same problem but just in case you might want to check that as well.