I forgot about it and went back to what I was doing online. Over the next 20 minutes or so, the noise continued, with me more or less aware of it. Eventually I got curious about this little, quiet motor again. Listening again, I realized it was coming from under my desk. It certainly wasn't my PC but then I remembered something else I'd spotted earlier.
I bent down to look, and there, in the shadows of the footwell, behind the footstool, a dark shape moved slightly. There was a faint gleam of eyes looking back at me. Diesel was down there, purring away steadily, for no reason except her own contentment.
People have often assumed that Diesel is male, named after Vin Diesel. Nope. She was named for her wonderful, diesel-like purr.