BambiBoom_Z,
Thanks for your reply.
1. Yes, it's 3.90 (June 2016). I've tested it with 3.88 before that, the results were the same.
2. Advanced > Thermal is set to the lowest fan setting
3. Yes. The driver's version is 368.81.
4. No. I've tested one graphics card after the other. With the Quadro installed, my Z420 remains silent (after the usual bootup fan test). With the GTX installed, the case fans of my Z420 run at a very high speed right after the bootup fan test (they don't run at maximum speed, but much faster than required).