I actually have the exact same noise on my 2010 roadster. Sounds like a plastic clip rolling around on the metal floor behind the drivers seat. I also thought it was in the convertible top storage cubby but couldn't find anything in there. I'm planning to remove the plastic trim behind the seat to find it. I'm sure it's a trim clip or harness clip that worked loose. Let me know what you find! Good luck!
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2010 370Z Roadster, Touring 6MT, Pearl White on Black, FI CBE 
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