It is the piece that connects to column to the gear and passes through the body in a rubber boot. It has a slip joint on it that binds and causes the popping noise. It will not be loose at all. The are not real easy to diagnose. The best way is to put your hand on it when you turn the wheel and see if you can feel it. If you have the front wheels off the ground when your turn the wheel the noise will go away as there is no load on the steering system.

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