I have had the whispering vent issues and I think I tracked down the leak. I found a hose running under the battery and it has a rubber cap on it and it looks like its stapled on. Its a larger hose not a small vacuum line but I put a rubber glove over the end of the hose (its ripped) and taped it and the AC seems to be blowing a little harder. What is this hose???

 

Looks like a drain utter

 

Yep, pretty sure that utter should be headed to the fender well just before the cove of the fender and behind the PCM. The utter is to the left in the pic. Pull the wheel and remove the acess panel for the PCM. You may also need to remove the screw that holds the bottom of the fender to the frame just before rocker. This would alow you to gently pull the fender out and carefully route the utter.
You might also want to check the vacuum tree and lines in that area. I'm pretty sure they control the acctuators for the hvac doors. You may find a solution to your whistling problem. Somebody moved the utter and may have not been so careful handling the vacuum tree and wiring.
I also see some possible leaking battery issues. Good time to clean up and paint the damage to the frame. Look the PCM and wiring over as well.

 

I can understand that the appearance of that "hose" may have been disconcerting. It in fact has nothing to do with the A/C but I can understand your possible confusion if you have not worked much in that area of the C5 before. But that part is not related to the A/C. That is a misrouted cowl drain udder. If you leave it taped up like that you will flood the cowl in the rain and the water may possibly work its way to the passenger compartment and damage things like the BCM. So don't leave it that way and while you are setting it back the way it goes, see the picture in the above post, now would be a good time to ensure it is not clogged with leaves and other debris as its function is to safely drain water away from the cowl area under the wiper arms.

As to your A/C issue. Has their been any recent work done? I usually see this after an intake has been off for some reason. The vacuum line that supplies the A/C system is usually left off at the back of the intake. The other main area to look is under the battery. Mounted on the fire wall near the outside of the car under the battery tray is the reservoir for the A/C vacuum system. Connected under it is the vacuum source for the A/C and the necessary tee's and check valve. You can also see the reservoir and white and black check valve in the pic in the above post. Check that all the appropriate hoses are connected properly at the reservoir while you have the battery tray off again.

 

It started after I bought it. Nothing done to it. Ill check the vacuum lines behind the fender and see what I can find. The battery leaked right after I bought it. Terrible luck.