I think what your getting is interference from the antenna of the iPhone, I have the same thing. Try putting tape over the top of the iPhone by the head phone jack, so that the metal headphone plug doesn't come in contact with the iPhone antenna.
I have the same issue with my iPod nano 6th generation, but never with any of my iPhones, iPod touch (4th generation), iPod nano 4th generation, iPod classic 7th generation, iPad 2 or iPod shuffle 4th generation.
I have exchanged my iPod nano 6th generation 4 times now (for different reasons) and the problem persist, not just with mine but with my friend's iPod nanos as well.
My point, I don't think it's an antenna issue but maybe Apple changed the headphone jack hardware from what they used to use in previous devices. Hopefully I'm wrong because I was considering an iPhone 4S but I use my phone as my car's media devic, I don't want that hissing noise.
I have 3 Aux cables and this issue (for me) only appears to affect my iPod nano.