Is the imac fan supposed to run all the time?

I just replaced the hd in my mid-2007 iMac with an ssd. I reinstalled the LCD screen and turned the iMac on to test it before putting the bezel back on. I noticed the fan in the bottom left is constantly running, but not high. It doesn't seem loud or anything, but just want to make sure I didn't screw anything up. I did reinstall the hd temp sensor as it was on the old drive. Now I noticed it did stop when I put the computer to sleep and started up again at waking it up.

The fans usually run all the time, at a slower RPM until temps start to rise then it kicks into high.