My Bose A20 are in the airplane but I did check my DC 13.4 that I had converted to ANR. I can hear a slight hiss in my left ear but not with my right ear. I have had degraded high frequency response in my right ear for many years.
I used those converted DC 13.4 in my PA-28 and was never aware of the hiss when flying. Much quieter than a running tap and really only aware it is/was there today at ANR switch on and switch off.