Why does my car make a crackling noise in the rear of the cabin?

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The problem occurs while driving (slow or fast speeds), and is especially noticeable when going over bumps or uneven pavement. The noise is in the rear of the cabin area and it is non-stop and sounds like as crackle or something rubbing together. I have torn apart the ceiling and rear side panels to check for loose wiring, etc. and also tightened up the housing for the interior "third" brake light but the noise still continues. It doesn't sound like a brake problem or wheel problem but I cannot locate the origin. Please help.