Does it change pitch or stop when you depress the clutch sitting stationary?
What about increasing the revs slowly stationary? any change...?
If fitted with Air-Con does it change at all when thats switched on or off?
What things have been carried out by the garage to try and track it down?
Ok.. Have they run the engine for short moment without the auxillary belt on? that could if the noise isnt there when thats done..confirm that something running on the auxillary belt is at fault maybe bearings on one the pulleys whether that be the tensioner or alternator,water pump even though that should generally be done at the same time as the T-Belt..might be worth trying out.....Going by your description i cant see it being a turbo related fault if it stops above a certain RPM......In that view the only other thing that comes to mind is the clutch release bearing if clutch depressed..maybe when stationary at idle apply a little pressure to the clutch pedal and listen to see if it does change at all,maybe sslect a gear and slowly bring the clutch up to near bite point and listen for any change..it can be quite diffcult to hear such noises over the engine..but if there is suttle change when depressing the clutch etc then that will be your problem i suspect......
Ok..it can be quite hard define noises over the net..but if you try the belt removal method then that will sort of eliminate that side of things on the engine....like i say its doesnt sound like a turbo issue to me..but if you wanted to go down that route of checks on that..then you would need to remove the intake tube from the compressor side of the turbo and then feel if there is any excess play in the vane shaft.you should have very little to no movement on that..and if thats case with yours then turbo is ruled out..that baiscally leaves either a problem with the gearbox/release bearing or input shaft bearing..sometimes using mechanics stethascope and listen in the region of the gearbox bellhousing might help diagnose noises from within there or the gearbox itself..