I think she's referring to temperature controlled fans. I've seen them on Compaq computers at least.

The fan starts spinning when the temperature inside the case of your computer rises. The airflow that the fan gives cools down the components and prevents your computer for overheating. It's perfectly normal.

However, if this is something new that is happening, I'd suggest checking the interior of the case for dust and blowing that out if necessary. If the fan is making "unhealthy" noises (this includes clicking, grinding etc) then you should replace it.

Otherwise, you're fine

GMAC