Mine is a '99 TD5 Auto, with 275k/kms on the dial. It is fitted with LPG which supplements the diesel with added power and improved economy. I recently noticed a reduction in power, most notable in cruise control as it changed down earlier, but overall not the power I was used to and reduced economy - 10litres/100kms not usual 8.5 litres/100kms. I recall hearing a hissing sound when it changed down but stupidly thought it was the aircon fan.
Culprit? a hole in the rubber elbow joining the outlet of the turbo to the steel pipe that leads to the intercooler. It appears that the pipe works on the rubber and cuts through. New rubber A$38, modified replacement, which includes a new pipe and rubber, A$245.
Winjeel.

I had a 99 td5 with turbo problems including lack of power and a hissing noise similar to what you describe in your post and the culprit was a faulty gasket on the inlet manifold. Hard to beleive as the car started first time every time and didnt smoke etc Might be woth a look !