Mine was in the same batch as well, and yes I can hear a very quiet background humming noise when no music is playing.
If I play music or video I can't hear the noise so I'm not too concerned.
Ahh ok thats a good sign, because I really think its just me wanting perfection but I can accept it if others have that! As you say its unnoticeable when something is playing. Oh and if you did wanna get rid of it, just double tap home press play on the music then put the volume to 0, then press pause and volume back up and it should be silent. I even think they may patch this...cos If I can do that it must be software