I went into the BIOS and checked the CPU Q-Fan Function. I have never explicitly set that, but noticed that it was set to Disabled (the default is Enabled). I then set the CPU Q-Fan Mode to Standard (also the default). It had been set to Turbo!. Saved and restarted and all is now AOK. Don't know how these ever got set to the non-default values.
Over the past few weeks, I experimented with some fan speed apps, but never (knowingly) changed anything. One of them was Speed Fan. Perhaps when messing with it, I unknowingly caused it to modify the BIOS. In any case, all is well now