Noisey cockpits I can't speak for the 757 but the 767 suffers the following problem.
The cockpit aircon and equip cooling noise reaches an unbearable level due the buildup of debris, dust etc in the ducts that supply the air.
I have seen the buildup so bad that the internal diameter of the duct is almost reduced to half capacity (imagine an old gal water pipe with rust inside).
As we all know air at subsonic speed is incompressable therefore to get the same amont of air thru the aircon/equip cooling ducts it must move faster and therefore alot more air noise.
Therein lies how to fix this problem, most of the cockpit panels need to be removed as it would appear the cockpit is assembled after and around all ducting installation.
Which means some heavy ground time. A luxury no airline wants to afford.