Mac Pro :: Fixing Never-ending Disk Activity?

For some strange reason, my early 2008 Mac Pro keeps making hard-drive access sounds / head-moving sounds. It's been doing it non stop for 3 days now. The sounds are short, about 3 per second, about as fast as you would read this: "grind grind grind" then a short half-second pause, then repeat. Sometimes there's a longer 1-2 second delay. But it almost always seems like triplets, "grind grind grind"

I have no apps or background processes running that I've been able to discover. I've checked startup items, launch agents, and launch daemons.

I've checked Activity monitor but I see nothing unusual, and no correlation with any cpu activity.

I've checked disk utility and SMART is verified, permissions are repaired, and the disk is verified to be "OK."

I've tried fs_usage, lsof, and top, and see no correlation to anything.

I'm clearly missing something and am hoping someone can point out what it is? What else can I do to determine what the disk activity is doing? Are there any apps or other UNIX commands to check? Or is my drive dying?