I looked into buying a ready-made NAS system, but all of them hard-mount the drives and have smallish fans. Hence my decision to build a chassis from scratch out of wood (not sure yet how I ill finish the wood, black like in the pic below, or just oiled/varnished). I'll suspend my 3 drives from the ceiling using bungee, with a 120mm fan at the back. Mini-ITX motherboard (Intel D201GLY) mounted vertical on the side (which hinges open for access to drives) with an HR05-SLI as cpu heatsink (which will be directly in the airflow from the rear fan. The OS will be on an IDE flash module (although you could quite easily use a 2.5inch drive or even a usb key. The PSU will hopefully be picopsu - although I'll have to do the sums to see if 3 drives will work on it.
I'm planning to use some linux NAS distro, connect via ethernet, and map a drive to it from my main windows pc.
Here's my design so far (made with google sketchup - thank you google!):
The wood should help against vibrational noise (and hopefully seek noise too), and the one fan will be running at 7 or 5 V so should be (as good as) inaudible. It should be fairly easy to do something similar for four 1TB drives.
The only downsides to this build are that there is no sata onboard (so I am using a sata card in a pci 33/32 slot, and there is only 10/100 ethernet. Since I'll be the only user and am currently running off one disk, this should be no problem for me. Besides that the price of the board + cpu (<60 pounds here in the uk) won me over
Be aware if you're considering a D201GLY that Intel are apparently releasing one soon with 2x sata ports and passive cpu cooling (according to this forum thread) so you may want to hold off getting the old one.