a few maint tips
recently our company went through the same problem with the 767 as the systems are the same and after alot of testing we came to a few answers.first check the recirc filters if they are old and full they increase the noise ,second some aircraft depending on line number actually have a noise attenuator or muffler fitted which can be retro fitted ,also depending on recirc fan p/n there are noisier fans ,most of the noise was actually airflow noise throught the overhead cb panel fitting the attenuator and ENSURING ALL DUCTING WAS PROPERLY CLIPED HELPED DRAMACTICALLY