There are in point of fact three fans in each of your iMacs. A CPU fan, a Hard Drive fan, and an optical drive fan. According to Marcel Bresink's excellent Hardware Monitor utility — on my iMac, which I believe is very similar to yours, the fan speed ranges are:
CPU Fan 940/2100 RPM
Hard Drive Fan 1100/5500 RPM
Optical Drive Fan 1000/3800 RPM
Run your hand over the vent near the top rear of the monitor and in normal conditions you can detect a slight breeze along with some significant warm air coming from the fans. Start burning video DVDs as I was last night and if you are in a quiet room and listen carefully you may hear the fans running.
