There's a hissing sound coming from the passenger side whenever I turn the A/C on. First noticed this on a rental about 2 months ago. I just purchased my Mustang a few days ago and noticed the same problem. Tonight I turned the A/C on and got under the dash, passenger side. The picture below is what I saw. When I put my finger over that round opening the hissing stops. Easy solution would be to put tape over it but I'm not sure if it needs to release air. I have no idea what this thing does but it's obviously a problem and I can't imagine why it doesn't have some kind of cover to silence the sound.

 

Take it in.