I have a 6" inline that moves 435 cfm. It was pretty loud until I hooked it up to my carbon filter and all all the ducting. That did wonders for the sound on its own. Probably not much help, but you should remember that in an apartment, there is no insulation on the inner walls separating rooms. However, there is insulation in the walls that separate the different apartments in order to cut down on noise, so it's probably not as loud as you think it is next door.
