I've used 2 4-drive boxes
a Netgear ReadyNAS and a Promise Smart Stor NS4300n
Both were relatively quiet, but not "audiophile" quiet.
I did modify the promise with a quiet fan as well as adding a low speed fan to the front to aid air flow.
the Netgear is quieter as a stock device, and has a variable speed fan. It can be loud when at full speed, but improves when idle.
the biggest issue i've had was a weird RAID failure with the Promise device where it would stutter with transfers, thus interrupting my music with pauses and ticks. Ultimately the device was rebuilding the RAID array, but non of the software monitoring tools were indicating a problem. IT has been working fine now for the last 16 months since the problem without any hiccups.
the netgear [purchased when the other box was on the fritz] has been problem free.
later,
todd