Airplane question ( OT Again)

[Doug Odom <dlodom@charter.net>]

Sorry about all these OT questions, but this group has the widest 
knowledge base that I know of.
A friend has an airplane and he has a problem with the noise level of 
the air coming in to pressurize the cabin. He tried a muffler type deal 
on it and it worked on the noise but  he could only fly up to 14K feet. 
Because of the restriction. If you have any experience with this kind of 
problem please e-mail me off list and  give me an idea or two. Doug Odom 
in big ditch