Thanks for the response, Kyllian.
When I boot, go into the BIOS, select Load Optimized Settings (or something that sounds very close to that) and save, I don't have this problem. (Actually, out of the five or so times I've done this, I did have the fan problem once, so although this technique does not seem to be a permanent workaround, it does suggest that the problem might be settings-related, but which settings I'm not sure.)
Does anyone know if there's software that could diagnose the problem?