I think the noise of the hdd could depend on where it is located relative to the front of the case. My Silverstone Temjin 6 (full size) places them at the back under the power supply. So it's pretty long way back in the case.
I can barely hear my Seagate when it seeks as compared to my old Dell where they placed the hardrives right in the front underneath the optical drives. I think most mid-size cases have to place them at the front under the opticals because of lack of space. I guess the farther back they are, the less you will hear them.
A comment was made regarding fans being louder than the hardrives. If your running a water cooling rig, then you will obviously want a quiet hdd since that's really all you will be hearing.