J. P. Gilliver (John) wrote:

So when I'm doing relatively little, and the fan suddenly ramps up and
then down again, I'm not left wondering "what caused that?"

To check the fan RPM along with temperatures (CPU, GPU, case), I use
Speedfan. There are probably other monitor-only tools but I need
Speedfan because the BIOS fails to control the CPU fan (always has it
run full speed). You could monitor the fan RPMs along with the
temperatures associated to them (CPU temp for CPU fan, etc) to make sure
it truly is a rise in temperature that causes the fan RPM to bounce up.
It is possible something bumps up the fan speed independent of
temperature.

I'll have to think about a CPU usage threshold monitor for a bit. Still
need some coffee.