All,
alexban- It has impact but next to the air running over the fuselage I doubt the AC is what you are hearing.
The noise in the cockpit is mostly due to the air slamming it's way up the front of the aircraft. The 747 has a huge noise problem due to the rapid accent of the air up the nose that drops down like a hammer on the upper fuselage. This occurs on all aircraft, but varies depending on aircraft type.
There are things that could be done and people are working on them.
The scary part is that if we made a perfectly "air noise" quiet flightdeck you guys would hear all the other clicking and etc noise that is below the ambient(due to wind noise) noise floor.
You have to understand, quieting a noise is only part of "noise reduction". As soon as you quiet one, another rears it's ugly head that was buried under the noise of the first and you couldn't "hear" it. Hence why most manufacturers are attacking turbo machinery noise on aircraft as that is the new frontier.
