Can someone tell me what this line is exactly for ( red hose )? I pulled the wiper blades off and the wiper cowls and all I can find is that it goes to a big black drum of some sorts on the firewall. I have some kind of leak some where back there that I can sometimes hear hissing.

No one knows? Yeah right, I need help cause this thing is getting on my nerves. Its starting to become a more of a high pitch whistle now but I cant find out where the leak is. I know it has to do with this line and I even used the extra hose stethascope trick but its such a high pitch whistle now I cant find it. Before I start pulling stuff out I ned to know what it is so I can prepare anything if needs be.

I think that when you apply the brake that supplies some vacuum or pressure to adjust egr or something. I remember hearing the same thing with my LS but was not able to stop it from making that sound.

My book is buried right now so I cannot look it up. But I do distinctly recall having to deal with the same condition you explained.

I have a similar problem with my 04 v6. I noticed the noise just after I got the car. Sounds almost like a turbo spooling up. A very high pitched whistle. After I got my KKM intake it was even more noticeable.

I believe someone commented on this problem before... suggested slightly boring out the red hose right where it meets the air intake... something about it being resticted there and causing a whistle.

see here:

http://www.lincolnvscadillac.com/showthread.php?t=29956