fan failure
Anybody experience fan noise in iMac, or fan failure?
iMac, Mac OS X (10.7.5)
You will need to be more specific about which iMac you have and what your are hearing.
First see > iMac: Evaluating System noises and reset your SMC if one or more is running at full speed.
1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.
Also note that there are 3 fans in the 2006 to 2011 Intel iMac's and only one in the 2012, 2013 models.
To check the rpm of your fan(s), get either > iStat Pro or Temperature Monitor