hissing through ipod

I hear a hissing sound through my ipod 30g and nano with the volume turned off. Does anyone else hear this?
I have a 60 Gb 4G iPod and a mini and I don't hear any hissing from either of them. Could be my 61-year-old ears, of course.
Maybe the Mini has hissing from electrical cicuit noise or I need a new sound card or I need contact cleaner. Sounds like tape hiss. Maybe need to go Apple store.