Generally speaking, I would not worry about that, unless you live in a very hot place. Your PC is designed to operate at 100% CPU usage and still dissipate enough heat to keep within the permissible operating temperature range. You may hear the processor fan increase in speed after a little while rendering - that's normal. Just make sure the airways are kept clear.

Assuming it's not a notebook, if it's more than a few months old and in a place with a lot of dust (e.g. if near or on the floor) I would suggest you open it up and (carefully!) clear out all the dust, especially from the processor heat sink and the processor and case cooling fans. You'll soon find out how often you need to do this.

It's incredible how quickly the grunge builds up even in what seems to be quite a clean environment. It's especially bad for smokers, even if they do not smoke at the workplace.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)