Anyone have any ideas what might be causing this sound....

Replaced the clutch in the ute 2 weekends ago. Used an 'extreme' clutch kit to go between an A15 and a 56A. Simply took out the old clutch, put in the new one, replaced the fluid and inspected the flywheel... did not remove the fly wheel.

First few days it worked a treat, now however after the car has sat a few hours (say overnight or while im at work) if I start it with the clutch depressed it makes a loud vibrating sound like a fan. The car starts just fine and if i leave it in neutral and release the clutch the sound stops, depress clutch and it starts again. Funny thing is if i whack it in gear and go for a drive (clutch engages and disengages perfectly) and then stop and try thr process again the noise is gone...... any ideas are appreciated??