Scosche passport causing hissing

I've used the Scosche Passport charging adapter in my car (2009 Infiniti) because later versions of iPhone and iPod use different charging pins in the 30 pin connector. My Infiniti follows older parameters. This works fine for my iPod and iPhone 3GS but not for my iPhone 4S. When connected in my car with the Passport between the iPod cable and the iPhone 4S, there is hissing and static noise on iPod music. Turning off wifi, bluetooth, etc, has not effect. Airplane mode has no effect. Hard reset of iPhone does not clear problem. Any help would be appreciated.