Here's my old PSU with modded with a Nexus fan @ 5v, soft mounted. I tried a Papst "9db" here first, and the Papst was quieter than the Nexus at anything above 5v. At 5v they both still click, but the Papst is the louder of the two. The PSU is also soft mounted to the chassis. The 120mm Nexus is also at 5v.
This is the mod that took the system from super quiet to silent. I had been running a 92mm Nexus fan here at 5v. It was nice because the fan itself was nearly silent, and the drives were stable at 30c, however the drives themselves were the main source of noise. I removed the fan and cut some foam to fit inside the front door. Voila! True silence!
View from the front with the door open, also showing the foam placed on top of each drive. The rubber bands help hold the foam tightly to the drives. Drive temperature is stable now at 43c. Much higher than before, but still safe, and much, much quieter -- no noise whatsoever from the Samsung, not even seek noises (AAM enabled). Placing foam on both sides of the drive did almost nothing to improve sound levels (which are now below the ambient noise of my extremely quiet room anyhow) but increased drive temps to ~55c.
Not the cleanest, but the case is closed anyhow, and there is no through-case air circulation anyways.
Cheers, and comments welcome!
Perhaps. But you can get away with a lot when you don't produce much heat to begin with. The Hitachi is off more than 90% of the time, and the Samsung is merely warm to the touch.
Advice: I use an wood box sealed all around my sistem. You can't hear it iven at nigth! Totaly silent! Excuse my poor english but it's not my first language. 
I seem to have succeeded. Temperatures are stable in the safe zone even under load, and I can't hear anything from this machine -- and I live in a very,very quiet neighborhood.