I think maybe it corresponds to the drive head moving, not entirely sure. I am probably going to buy some noise suppressing materials for the inside of my Antec 900 case, see if that will take away some of the noise
I have a 74gig Raptor and the 150 Raptor X in an antec 900 case next to my monitor, less dirt from the carpet this way getting into my new case, but I can honestly say, i don't hear the drive that much, unless I am doing a lot of read/writes to the hard drive.
If you are hearing a ticking/clicking sound, use the WD diagnostic software to test it. If it doesn't show anything wrong, I would monitor it for a month or two, just to make sure.
My Antec 900 case isn't that loud, maybe I just used to the loud noise from my other case, that makes the Antec seem very quiet. Don't know.
I do know, my 150gig Raptor X doesn't make a clicking/ticking sound, unless I am doing read/writes, and then, in the new antec case, it isn't that loud.