Clutch pedal vibration

Zafira diesel 1.9 cdti problem with clutch vibration. Car drives fine, no problems at all however when I start the car there is vibration in clutch pedal. Press in clutch pedal 1/2 way (sort of bite point) there is like a loud hum noise in engine. Pedal to floor no noise just slight vibration on pedal. Release clutch pedal and on bite point again same hum noise and vibration. Gears work fine, plenty of brake/clutch fluid in reservoir and no obvious leaks around. I was thinking release bearing? Could this be? Thanks.