Pssst or Hiss noise when changing up the gears

It's a leak in a vac.line or one of the clamps around the turbo boot is loose or split either on the high psi side or low (intake) side.You can tell by the way the car idles if it's running smooth at idle the it's the low side if it's rough then it's the high side of the turbo side.

Many thanks for that, I'll take a look. Do you know of any web sites that will show me exactly where to look - E.g. a schematic of the engine bay??

Not sure if this is relevant but does the LPT have a Dump Valve on the Turbo? if so it could be this that your hearing as it drops the waste boost? if not you have lots of 1/8th bore vac. hoses, check these especialy where they terminate or join, my wifes split a a "Y" joint & exhibited a loss of boost on the turbo/apc gauge.
Regards paul

Yes, I can confirm that the LPT does have a dump valve.
In line with the advice given, if the car runs smoothly, with no performance problems, then it's probably just a noisey dump valve - so no problem! Otherwise, applying the tes wrt the first respondant is a good place to start tracking down your air leak!

My standard '96 aero gives a nice little hiss on changing up when accelerating quite hard. The car is in vgc (done 70K) so assumed this was normal but do you think this could mean I have a holed hose or dodgy dump valve?