I have not been on the Westerdam but the general rule of thumb is that the noise associated with elevators is people talking, laughing and having a good time. The stability of the ship is excellent. If you are speaking of where you sense the least movement, it would be the lowest deck, and midship.
On the Westerdam is there distruptive noise if your cabin is by the elevators at forward? Also is the stability on the ship good if your cabin is forward?
Avoid being in a room too close to the elevator lobby. Noise from people has been mentioned, but the most annoying thing is the "ding ding ding" of the elevator bell as it reaches your floor. That goes on all night.