re: Diagnose this noise my hard drive is making

My bet is on thermal recalibration, which is normal and fits the pattern you described, especially if the drive is being regularly accessed which would momentarily interrupt a potentially lengthy recal. Google the make and model number to see if anyone else has commented on the behavior.

It could also be a background process at work, and if so, you should see the drive activity LED flicker, and if you're lucky, you might be able to catch it in the act with a program like Process Monitor, though it might stop as soon as you access the drive. There will be no LED flicker for internal drive operations like thermal recal.

If you're not showing any SMART errors in CrystalDiskInfo or disk or NTFS errors in Event Viewer, and you're not noticing any freezes or slowdowns, I'm not sure I'd worry about it.