I managed to record it using my laptops built-in microphone.
To those who would prefer a description:
It starts fast and slows down. I opened the file with Audacity and determined the intervals to be 0.1, 0.3, 0.6, 10, 15 seconds, then it repeats.
It sounds like some kind of protocol negotiation algorithm that requires a disk sync when the polling time doubles.
Here's a link for your listening pleasure:
I'm hoping it's some cron job, or DropBox, Trinity Desktop, wifi, smart disk status monitoring, nVidia, Intel, ... not a virus trying to phone home.
I'm really hoping that someone reading this will go "oh that thing" and know what it is instantly.
Curiously it doesn't happen every day.