If what you are hearing is fluctuating seek noise, as opposed to a constant whine, you can probably reduce the noise significantly by changing the drives from the "performance seek mode," which they are probably set to by default, to the "quiet seek mode."
Western Digital Caviar Black (and other) drives provide both of those modes. Remarkably, however, as far as I know WD does not provide a means of changing which mode the drive is set to. In order to change it you have to use a third party software utility. WinAAM is one such utility, which you can find linked to and discussed here
. You can probably find other such utilities by Googling "hard drive acoustic management utility."
For this kind of utility to be able to "see" the hard drives, you might have to temporarily remove each of them from the RAID enclosure and install it in a computer as an additional internal hard drive. I would not do that, of course, without first backing up any important files they may contain.
Disclaimer: I have no personal experience with WinAAM or any other such utility.