Tritone

So I get a hiss from the rear that gets louder when I accelerate up hills but only above 80kph and slowing goes away when going downhill or level road. The hiss remains after the car is off but slowly goes away after a couple minutes. 
sounds like a puncture but is not.
Any ideas?

2008 2x4 petrol Manuel Hilux

So I get a hiss from the rear that gets louder when I accelerate up hills but only above 80kph and slowing goes away when going downhill or level road. The hiss remains after the car is off but slowly goes away after a couple minutes. 
Any ideas?

2008 2x4 petrol Manuel Hilux

So I get a hiss from the rear that gets louder when I accelerate up hills but only above 80kph and slowing goes away when going downhill or level road. The hiss remains after the car is off but slowly goes away after a couple minutes. 
Any ideas?