A high pitched whining noise when the clutch is pressed down, what would cause?

POSSIBLE THROW OUT BEARING. IT SPINS WHEN YOU DEPRESS CLUTCH

If it is first start up over night I have seen it alot on the Pathfinder and Frontier where the $8, pilot bearing behind the flywheel goes dry and fails. You would replace the release bearing anyway and probably the crank rear oil seal and clutch assembly unless all the parts looked perfect.