I've noticed this previously in my 2011 Z. Haven't bothered to screw with it because it's inconsistent and infrequent. Plus, I don't have passengers very often.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |