Brian,
As I mentioned, above, I purchased a 250Gb WD drive to use as my new internal media drive (Mac 1GHz DP) to replace a 120Gb IBM DeskStar (which now resides in an external FW enclosure). I installed it last week and all has run smoothly and quietly.

I couldn't help noticing, however, that the "clicking" sound is noted in the troubleshooting FAQ in WD's instructions. Here's, in brief, what they say.
They then instruct to power-down, disconnect the data cable, and then power-up. If the clicking continues, they would conclude that the drive has failed. If not, they suggest that a faulty data cable may be the culprit.

Glad to hear that all's finally fine at your end.