Minh,
I believe what you are hearing is normal.
The piano uses solenoids to push the keys up from the bottom back side of the keys. Some of the noise you are hearing are the solenoids, some of the noise is the normal mechanical noise of the piano action.
The softer the piano is played, the more this noise can be heard.
Sent from my iPhone.
Bill
On my new DGB1KE3 I can hear a combination of minor squishing and thumping sound from the moving key mechanism if I really pay close attention to the keys on quieter musical passages. From a few feet away or when the volume is louder, however, it is practically unnoticeable. I wonder if anyone observed this?
Minh