The ventilation air intakes are forward of the cockpit on both side of the fuselage. Guess this leads to some "tunneling" effects and the vents are noisy.
The airplane is surprisingly quiet with the vents closed.
On the subject of active noise reduction headsets and Diamonds...
I used to have a Telex 50 ANR, couldn't use it in the Diamonds.
For some reason ( I suspect the shape of the canopy) I always got interference when I switched it on.
Buddy of mine bought the new David Clark ANR, same story, so he switched to the Bose headset also.
In my humble opinion the Bose is one of the very best headsets ever made, expensive though.