I filed an RMA with Western Digital for the same "issue". I received a replacement drive which also makes the same mild clicks occasionally. Replacement drive was running an older firmware version than the one I bought which means they were out of the factory in different batches.
I've run short and extended drive tests using the WD Drive Utilities software. Everything is normal. After copying over 300 GB in each drive, SMART does not report anything abnormal (no errors, no failures).
I'm keeping one of the drives and returning the other one according to the advance RMA agreement. I'm now convinced that the worrisome sound is nothing to worry about :)
It cost me a week in time and $10 shipping to put my mind at ease.
P.S.: It seems that the clicking sound occurs when there is disk activity, reads or writes. I have the impression that it happens mostly during writes but I wouldn't bet on it. However, it seems there is no effect in transfer rate or anything else.
P.S.2: I really do own this exact drive (2TB) model. If anyone said to me over the phone or in some forum that his drive is clicking I would say it's defective right away, like the other guys told you. My guess is though that they do not own this model and are thus drawing general conclusions.