What I found included in the DS6 was an optional fan/radiator bracket that could be mounted against the drives on the inside of the case, making a total of four 140mm fans primarily devoted to cooling the drives.
Does the DS5 come with a mounting bracket like that? When configured that way, I found I could turn down the drive speed to Nanoxia's minimum setting and still get adequate drive cooling. At that point, the noise from the nine hard drives starts to become the dominant noise source, but I'm not sure what more, if anything, can be done to reduce that other than possibly setting the drive firmware to WD's so-called "accoustic mode".