I was driving today and heard a really weird noise, couldn't figure out what it was and then stopped and listened close to the center vent and there it was. A hissing sound, I remember reading a thread and seeing something about a hiss, but can't remember where i read it, or what it was about.anyone else know what I mean?

 

I've had this happen to me and realized that I had closed one or more of the other vents. When I cracked the other vent slightly, the hiss went away. Not very technical, but sometimes we overlook the really easy stuff.

 

I actually have that problem right now and they said it was part of the airconditoning problem.

 

I had this issue, and the dealer told me it was common on 99 E320's. However, I looked up old posting, and they say to have the A/C checked for leaks and recharge any coolant if low. Well, there were no leaks, but the coolant was 1.5lbs low. After the recharge, no hissing. It's been 2 months, so far so good.

Good luck.