Hissing sound under dash?

[zeuss000]

Hi Folks, I have a strange problem with my AU2 Fairmont Ghia, as the title suggests there is a faint hissing sound under the dash somewhere. It starts as soon as i start the car, sounds like a leak which i'm sure it must be, it doesn't vary when i turn the fan or air cond on, just stays the same volume and pitch. When i turn the car off it keeps hissing for about 20 secs then fades away. I can't see anything under there that it might be, i have no idea what it is coming from. Any suggestions?
Thankyou and MERRY CHRISTMAS everybody.
Ed