Most people will categorize crashing, overheating, and graphical artifacts as a more severe problem than "what's that small noise I hear once in a while".

> no discernible reason

What if the "no discernible reason" is just poorly coded software?

That's the definition of a discernible reason. If the CPU is being used hard enough to need more cooling, then the question is answered.

That's too limited a view at the issue. Why is the CPU running that hard? If the user is not doing anything CPU-intensive, yet the fans spin up, clearly the CPU is hot, but why? What is it doing?

Then the PC and the OS is off the hook and the search continues with the software.

It’s not. Using the cpu is a supported feature of normal computers.

If you can say "the fans are spinning up because this piece of software is using the CPU more" then it's not "no discernible reason".