I would say that the X3 is pretty loud for inside an apartment.
You probably will have the neighbors up in arms, But if you put it in an enclosure and sound proof the enclosure you should be able to get away with it. And if you are cutting aluminum, the enclosure will keep most of the swarf inside the enclosure.
You will have to get creative on chip collection if you have it in your apartment or the chips will migrate all over your apartment.
I can have a normal level conversation with someone right next to the mill if the enclosure is closed, but if its open I would think its too load for an apartment.