Any idea what this sound is?
It’s the “Inactivity Sound,” called “Bass,” that Assistly plays by default when  you haven’t been using it for a few hours.

And I thought my computer was overheating!

I can’t understand why anyone would make a web app that non-obviously plays a repeating, scary sound on repeat hours after the user last interacted with it.  

Terrible.

Anyway, that’s one mystery solved.

You can +1 my relevant feedback to Assistly here.