Based on your symptom, I'd vote for the input shaft bearing. With the clutch pedal on the floor (clutch disengaged), the input shaft isn't turning - the only relative motion is between the input shaft and the pilot bushing, and the throwout bearing is rotating. When the clutch pedal comes up and the clutch starts to engage, the input shaft spins up, and when the pedal is all the way up, motion stops between the pilot bushing and the input shaft, and the throwout bearing is off the clutch fingers and stops rotating.