When my clutch is out I get a rattle between 3000-4000 rpm. When the clutch is in, then it doesn't rattle. A mechanical friend suggested it could be the thrust bearing on the clutch.

Has anyone had any experience of this, if so I would like to know how much it should cost me for the repairs.

 

I think your friend is right.
My clutch doesnt rattle too bad but you can just about hear it.
My clutch also whines a little when the car is first fired up in the morning but this clears up after a few seconds.
With clutches costing neally £500 i am gonner leave mine till it gets really bad.