I should have qualified that in that there is probably some psychomatic effect there. It's a very slight perception, but I've noticed on every ENC headset I've tried, DC, Bose, etc. Maybe it's just me being paranoid.
Anyway, when you have the headset on and you add power, there's kind of a wump as the noise disappears.
My theory goes like this: ENC/ANR works by reversing the wave form of the ambient noise. It adds that reverse wave to the inbound radio/intercom signal. If this reverse wave exactly matches the frequencies and amplitudes of the abmient noise as it would have conducted into the earcup, the noise is 100% cancelled. There is no 100% in science or technology. So, if the frequency is of just a hair, the sound may be reduced overall, but there is now an interference sound that could be produced. Now, this is my theory: the resultant interference noise is now above the range of human hearing. High frequency noise causes low frequency hearing loss (a fact). The noise cancelling properties of these headsets may reduce the noise we hear now, but may be causing hearing loss down the road.