Yeah, you'd think hard drives could last a little longer than a year or two. Oh well, as I was diagnosing the problem I suddenly remembered that I have another drive of the exact same model from when I was running my system with RAID-0. So I swapped the noisy one out for a quiet one off the shelf. In a few months I plan on building a second system, so we'll see if the drive gets better with a little R&R.