Korg PA1000 key release noise

[fpa1000r]

Is it normal that for a few of keys mainly the C keys, it makes some kind of noise like spring shaking when the key is released naturally? Something like https://www.youtube.com/watch?v=ZWplHbi6t0U
Is this a random thing and could happen to anyone or this is actually a defect?

Once I started to notice this, I can't stop it and it distracts me a lot whenever I hit the key...

[fpa1000r]

To answer myself, I called Korg directly, they said this is normal and can't be repaired because of the design, even if they replace another one for me, it will have the same problem.
I can't believe this is happening on Korg.