For years I heard rattling sound from behind my dashboard. It has a tendency to get worse with time. At first, it was faint rattle, then louder, and now sounds like something has gone completely loose, as in the sound you can get by shaking a plastic box with a screw inside. It's roughly coming from the area behind the instrument cluster. When this rattling sound is present, sometimes taping on top of the dash board, next to the top driver side AC vent ends it. When I was gluing back the detached dash material, I had take off the black plastic cover to apply the glue, and back then, I couldn't locate the source of this noise even though I tried tapping on every surface to see if it would rattle. Any ideas what could be causing it?