Why can I sometimes hear the rotor moving when I turn my wrist?

That's the rotor spinning, totally normal. It's an automatic watch, right? The rotor winds the mainspring when you move.

Yep, automatic movement. The rotor is a semicircular weight that spins as you move your wrist. On skeleton watches, the rotor is often visible through the dial, and since there's less material, you might hear it more.

Totally normal. Some rotors are noisier than others. My Seiko 5 is pretty quiet, but my Orient Star can be heard if I shwke my arm.

I have a skeleton watch too and I hear it sometimes. I was worried at first but it's just the rotor. As long as it keeps time fine, don't stress.

Is it a unidirectional or bidirectional rotor? Some rotors only spin one way, so they might make more noise when they spin freely in the other direction.

Good point. Also, if the watch has a ball bearing rotor, it can be smoother and quieter. But some cheaper movements use a pinion and can be noisier.

I've had a few automatics, and the skeleton ones do tend to transmit more sound because the case might be thinner or the movement is less enclosed. It's part of the charm, I think.

If it's a rattling sound, could be the rotor bearing. But a whirring is fine. You might also hear it if you flick your wrist hard.

I wouldn't worry unless the watch stops or runs erratically. The rotor noise is just a byproduct of the automatic winding mechanism.

Does it happen when you're wearing it or just when you pick it up? Because if it's just when you pick it up, that's normal since the rotor can spin freely.

Mine does it when I put it on in the morning.I think it's because it's been sitting still and then I move it a lot.

Also, the rotor might spin more freely if the watch is low on power reserve. Once it's fully wound, it might not spin as much. But it's still normal.

If you're really concerned, have a watchmaker listen to it. But from what you describe, it's nothing to worry about.