Question about Bose QuietComfort Earbuds

If I set the noise reduction to level 0, I can always hear a slight hiss. I think this is because the microphone is then active. Only below level 10 does the noise disappear completely. Can anyone confirm this behaviour? I find the noise annoying, especially in the evening when I'm listening to a lecture. In this situation, ANC is unpleasant (in only one ear).