Clutch Tranny problem?

Hey folks,

I've got a 1972 Volkswagen Super Beetle. With the engine idling and the clutch engaged (peddle in upward possition) while in neutral a humming sound can clearly be heard until the clutch peddle is depressed (in downward possition as if to shift gears), then the sound fades out and silences. Any ideas as to what component(s) could be causing this hum?

Thanks,
Matt

throw out bearing and pilot bearing