I noticed after the most recent Windows10 update that my Surface Pro was running continuously at 15% cpu usage even when idle. Causing the fan to start and stop at random intervals.
After some digging it turned out to be caused by a screensaver slideshow running on the login screen. Or at least, turning that off cured the problem.
Which adds two more items to the long list of Windows mysteries. Why leave a login slideshow running in the background after a user has logged in? And why change it in the latest update?
To turn it off, go to Start > Settings > Personalisation > Lock screen. In the drop-down change Slideshow to Picture. Or alternatively scroll down and switch off "Show lock screen background picture".
Your mileage may vary. May contain nuts.
Martin.