Have the car stopped and running and have someone turn the wheel back and forth while you feel the right tire. Mine makes a click/pop as well and I believe it is my ball joint. You can feel it through the tire if it is as someone turns the wheel.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000