i have 2 wd 120g satas. last install (as is this one) they were raid 0, and i got the clicking sound every so often. i thought one of my drives was soon-to-be toast. it sounded like there was play in the spindle and every time it spun, it took up the slack with a click. or the read head was making contact. that's just what it sounded like.
the problem was with a pin in the molex. it lost it's barb and was loose. whenever i went in my case and pushed on it, the clicking went away. since then, i did away with the molex adapters as i got a better psu with sata plugs.
with a fresh install, the clicking went away and i've been running them problem free for alittle over a year now. god, i hope i didn't just jinx myself. i'm gonna backup right now.