antiquer,
the hissing sound is for sure from the rubber seals around ur windscreen. to know where exactly the sound is from, do the below:
1. tape with masking tape all around your windscreen covering all the rubber seals. go for a drive. you will realise the sound is gone.
2. remove the tape bit by bit probably 1ft or so each time. and everytime you remove a little.. go for a drive untill you hear the sound again. then you can pin point where the sound is from. I have done that for all of my 32, 38, 39ss(x2).
in anyway, you might need to change the rubber strips.