Well, after all that you've got to check a few more things. The steering rack and the tie rod ends are possible sources, especially if the rack is loose and moving back and forth. Suggestion, get the car up on a lift and have someone cycle the wheel back and forth quickly. You may be able to notice if anything is moving around or making any noise. Ball joints are another possibility, but I'd bet that it's probably the rack, tie rod ends or the steering linkage u-joint(from the steering wheel to the rack) Hope this helps..
1987 BMW 325