Suspected clutch or gearbox problem

There is a brief (2 or 3 seconds max) squeaking noise just after releasing the clutch. It only happens when changing gear upwards and nearly always in gears 3-6. It never happens when vehicle is stationary or when changing down. It also only happens when the engine has warmed up. Original diagnosis was clutch release bearing, but this has now been replaced along with the clutch plate and flywheel. The squeaking noise is still happening in the same conditions i.e. after clutch release when changing up through the gears after engine is warm. The expense of work done so far and no resolution is worrying to say the least. Any information on what the problem could be would be very welcome.