What is causing slight clicking sound coming from rear of car when moving?

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Over the past several months I have noticed a slight clicking sound coming from rear of car. Only occurs when car is moving forward at lower speeds, higher speeds, sound is inaudible. I have had several mechanics look and listen for it, none of them have been able to locate the origin of clicking sound. According the mechanics, it isn't shocks although, one shock bushing was replace as a possible cause two weeks ago, not the problem, $300.00 later.