high pitch hiss comming from rear of car?

[Orange Vert]

Sitting in the drivers seat my 99 GT 5 speed I can hear a high pitched but quiet hissing sound coming from the rear of the car. It gets louder as the speed increases. The sound doesn't change whether it's in gear or out of gear with the rpm's dropped. It's in relation with the speed. I thought maybe at first an air leak in the back window but it's too constant. Any idea's? Thanks.

 

[ZincGT]

Do you have aftermarket gears?

Could be your differential. Whether you have gears or not, any work that has been done poorly on the differential can result in it being noisy.