I can hear an audible hiss when using the HM-101 and listening out of the Sony MDR-7506 via the headphone out. The said hiss comes in when the DAC is engaged, meaning it's not always there. For example, when playing music in foobar, when I press the play button and pause it, the hiss is there, but when I stop the music, it disappears. When watching streaming videos, it's there when the video loads, even when I pause it, but disappears after about ten seconds of inactivity.
The hiss is very weak and easily forgotten when there's a normal intensity of sound playing, but it's definitely there.
