A little surprised the 85/1.8 is quieter than the 40mm.... isn't the STM supposed to be a quiet motor?
STM lenses make a high pitch noise, almost similar to a loud version of baldy shielded processor noise leaking into amplifier circuitry. Nothing like the volume of micro motors of older cheap lenses. It's almost completely silent, and great for video work. USM motors are silent to all intents and purposes, although there is typically the noise of components other than the motor - the internals of the lens gliding over each other etc. And of course there's the fairly abrupt torque as the AF system start/stops to achieve its fast focus acquisition speed. This is no good when it comes to slow, smooth video AF requirements as the USM system will go through a constant start/stop sequence.
So - for video with DPAF, STM is significantly quieter and smoother (both for focus smoothness and torque/recoil you feel as you hold the lens), whereas USM is a little quieter and typically much faster for images.