If i dont push the car at all then you won't hear anything but when i accelerate hard in 2nd 3rd or 4th (i havent tried 1st and prob cant go fast enough in 5th) i hear a hissing noise that gets progressively louder (but not particularly loud) as the revs go up. It take a while to show iteself, again i can drive around quite steadily and never hear it. As soon as i come off the power it stops. It almost sounds like air hitting the car at a funny angle on a blowy day.
Any ideas?

Anyone? Not that hard for the experts surely?

have you checked all the rubber vacume hoses for splits or leaks,also check around the injectors for signs of diesel as its common for the injector seals to wear.