Hi David
Its always difficult to daignose from sounds, but IF it is the clutch release bearing, you will get a noise when the engine is started and its just idling in neutral, the noise a clutch release bearing makes is a whirring type noise (unless it collapses all together) , the easiest way to diagnose a release bearing is as follows, with the car started and in nuetral listen for a noise (i would say a whirring rumbling noise) then press the clutch pedal to the floor, by doing this ,the noise should decrease or go altogether. the noises you describe (in my opinion might be something different,possibly related to your clutch plate itself)is the biting point at the top? does the gear change smoothly in all gears inc reverse?
If it is the release bearing or clutch obviously its a gearbox out job and unless you can do this yourself, will be fairly costly, more in labour than parts.
obviously any noises coming from the gearbox/clutch is a concern but any car ive owned WITH a clutch release bearing on its way out, (although sometimes very noisy) has never failed on me, maybe ive been lucky but ive driven them for years without any problems. (im not in any way suggesting you do the same as the diagnosis has not been confirmed)
hope this helps .
mark
