Clutch release bearing is part of the clutch, not the gearbox.
Does the noise happen when the car is sitting idling in gear? If so, the selespeed system will have the clutch disengaged and so the gearbox will not be turning at all, so your problem is unlikely to be the gearbox.
If you leave the car for at say an hour and then come back and open the drivers door, does the car make the noise for 5 seconds or so? This is the selespeed pump priming the system and if the funny noise is evident then, your sele pump needs replacing. If however the pump hums away quietly for 5 secs or so then all is well and it's probably the clutch release bearing.
Just because the previous owner said there was a gearbox problem doesn't mean there is necessarily a problem with the gearbox itself - could be clutch, gearbox, or selespeed system.