1995 Nissan Maxima Whirring noise when clutch is engaged

When clutch is released when idling in neutral (and when engaging gears) a whirring noise is heard. When the clutch pedal is depressed there is no noise. As the car is driven and warms up the noise persists, but a lower level. I can imagine needle bearings disentregating or not getting an oil bath (the tranny is full). The car has a hydraulic clutch which, I've been told, cannot be adjusted.

Any clues?

Thanks a lot.