I just bought a WD Caviar Black 1TB 2 weeks ago. Sometimes I can hear the hard drive clicking occasionally like once ever 30 seconds - 1 minute either at idle or under load. However, even when its under load everything seems to still be working and there is no freezing whatsoever. Usually the clicking won't stop until I do something completely different from the task that I was currently doing (Stop gaming, or actually start a file transfer just to make it not idle)
The only reason I ask if this is the infamous CoD is because it doesn't sound like a normal hard drive "click" sound, it sounds kinda metallic. Also as stated above it only occurs occasionally (like once a day or something) and it'll happen either under stress or idle with no freezing occuring.
Also, I have enough power running through this (1000w PSU) so it can't be a power issue and I've check all the connections to the hard drive. Temps are even under 28C under stress.