When both the carbon and roller bearing types give up too soon, it is likely the clutch hydraulics are not releasing pressure as they should. With the carbon one, a constant riding action on the pressureplate is there by design, but the pressure (from the spring inside the SC) should be light. Some roller ball bearings fail with this light pressure, but the double one with both a carbon and a roller bearing part on top each other is more resilient. Still, in case with a rollerbal one, modifying the clutch actuation mechanism with some kind of pullback mechanism is best when using a roller TO bearing of any kind (I myself and RAY are using this).
Even if going back to carbon I suggest you at least renew the seals in the clutch MC or the whole unit, so the hydraulic fluid at least is free to flow back.
PS Rumbling is not what comes to mind in case of TO bearing failure. How I remember my carbon TO bearing failure was a very uneven clutch action, making the engagement of any gear progressively difficult as the carbon pieces dropped out of the ring one by one (I only just managed to reach home)
A slight rumble when no gear is engaged is completely normal: the first motion shaft is being turned by the engine without any gearwheels engaged, so makes noise. It should disappear when the clutch pedal is pushed in, bringing this shaft to standstill, by which this slight rumble should go away
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