iMac 21.5" late 2013 users: do you ever hear the computer's fans make noise?

I used to have a late 2006 iMac up until recently. I never heard its fans.
Now, with my new late 2013 iMac, every now and then the fans kick in for a few seconds (during those few seconds I can clearly hear them) and then turn off and the computer becomes quiet.
Is that normal behavior?

By the way, CPU usage during those events is about 5%. Apps that are running are Safari, iTunes, Mail.

Thanks for your responses!

The fastest way that I can make the fan on my mac mini come on is by playing videos. I think full screen puts more demand on it as well. I also have a MBP that I can't hardly hear when the fan is on.