new gearbox is whining

[Slamz]

I'd change the release bearing while it's off, just to rule it out. It spinning freely isn't enough to indicate it's not noisy.

[Zornyan]

You need to spin the release bearing in your hand and feel for defects in the motion. Hard to explain but even a tiny notchyness or odd feel in part of it can be a loud noise at speed.