The nice thing about unraid is the array needs to only spin up the disks that are being accessed AND the parity disks (only while writing). I run two parity disks so that I could lose two disks in total before the array fails. While running a parity check all drives get spun up and the total noise level is arround 35db but this totally depends on the drives you are using. For me it's mostly Western Digital Ultrastar DC HC530 shucked from WD Elements because of cost savings.