Rio has confirmed that audible static may appear when using certain kinds of headphones with the new Rio Carbon. Apparently the static is produced by a headphones with a metal base coming into contact with the metal surrounding the headphone jack. Pertinent Riovolution forums threads are here and here. Rio's official statement is here:
Q:I hear Static in the headphones I use with the Rio Carbon
A: A small number of users have reported that when they use their own headhpones, not the ones included with the Carbon, the sometimes hear static during playback.
[Via DAP Review]