[A Quiet Boom]

I also agree with the larger/slower fans suggestion. I have a Lian Li aluminum server case that came with a total of 4 120mm high speed fans. Since I'm running relatively cool running quantum raid drives (8 total) and a quiet and cool for it's speed Seagate 15k RPM drive I went to slower fans and dropped the noise substantially. On my server I built the case myself by essentially bolting two cases side by side with one side for the Raid arrays, I used Dynamat extensively in that case and really quieted down the noise. Dynamat is a self-adhesive sound deadner used by the car audio guys. Despite the fact that the machine has 12 older and really loud SCSI drives the dynamat reduced the noise to that of a regular computer. I've also become a fan (no pun intended) of the newer CPU heatsinks that use 80mm fans, I built a comp for my dad with one that I installed a two speed switch on, 90 percent of the time it's on the quiet low speed. Another great solution I use is what I've done for the comp in my entertainment center, I put it in a Wood cabinet and added front floor air inlets and rear exhaust. With the door closed you cannot hear the computer. A cheap Lowes or Home Depot cabinet works really well for this or you can build your own. One of these days I'm going to rebuild my home server using a custom wood case and one large squirrel cage blower for exhaust.

Water cooling should be a last resort IMO.