The main sound of the fridge is generally the sound of the 150mm fan. You could replace the fan with a smaller and more quiet fan but then your fridge would probably not work as efficiently. The brushless 150mm fan is the same type of fan in both the Series 4 and Series X. You could double check the screws that hold the fan to the roof of the unit are tight as a loose fan would rattle a bit and this could possibly cause some additional noise. If wires or anything else come in contact with the fan blade (or built up ice) this can make quite a loud sound. This fan is not dissimilar to the fans used inside a desktop computer case. So the sound of the fan would be similar to this level of sound.
I should also say that both models of fridge have been made with exposed evaporator. A very small amount of noise (generally less than the fan) is made by liquid refrigerant gas entering this evaporator. Some modern fridges have covers over the evaporator or plastic shroud that covers the evaporator. A plastic shroud like this might help muffle the sound but it also would take up a bit more space and also would not make the fridge as electrically efficient as having the evaporator exposed. So you would pay the price very slightly in electrical efficiency. It should also be noted that we use the r600a which is very slightly more noisy than the r134a. The R600a is also more electrically efficient and environmentally friendly which is why we use it.
So the electrical efficiency does come at a slight compromise when it comes to sound but I would not call the
Series X a noisy fridge at all. It's definitely more quiet than our
Grand Deluxe ones that have multiple radiator fans running inside this unit.