Rhythmic whirring sound from my Dell Inspiron

I am getting a rhythmic whirring sound from my computer; a fan I'd assume. It occurs very regularly (about once every 10 seconds). You hear the fan ramp up and ramp down for about a second, followed by 8 seconds of quiet, just to repeat.

I don't have a CD/DVD in the drive. It doesn't matter whether I'm using the computer or it's sitting idle. I figured I'd pull it out, open the case and listen. Hopefully what to replace will be obvious, but I'm looking for any insight first.