iMAC fan is making a very loud noise

It can be caused because of different reasons: a process which is eating your CPU, a defective fan, a problem with the SMC, etc. First, reset the SMC > http://support.apple.com/kb/HT3964

If it doesn't work, run Apple Hardware Test > http://support.apple.com/kb/ht1509 This program will allow you to diagnose problems with the hardware of your computer, although this may fail and it may not show anything. When you run Apple Hardware Test, make sure you tick the box to run an extended test. Finally, if you see an error at the end, copy it here and copy it in a piece of paper and take your computer to an Apple Store or reseller, because it indicates that there's a hardware problem with your iMac and it must be fixed

Its because the temperature sensors are not working and the fan is being running on FULL THROTTLE, full rev's .

Best solution ( FREE), just download and install : http://exirion.net/ssdfanctrl/

1. Download the package

2. Run the installer (reboots your iMac after finishing)

3. Run the SSD Fan Control application

4. Find the noisy fan and control it in ‘Manual’ or ‘SMART’ mode

5. Enjoy the sound of silence

Thank you for this link! Works beautifully.

I just had my mid-2011 27" iMac i7 in for a service and decided to switch out the back-up data drive to a new SSD to complement the original OS SSD. When I started up the machine afterwards the cooling fans were running non-stop, which is an especially offensive issue if your machine happens to be a dedicated audio work-station, as mine is, and you have gone to great length and expense to create a suitably prepared acoustic environment in which to work.

I was relying completely on the expertise of the professionals I took my machine to, but I still did ask what I thought was a stupidly obvious question: "Will the new SSD (which turned out to be a Samsung) interact with the OSX in exactly the same way as my old Western Digital SATA"? Which it does of course, but it seems that there are some underlying issues with the propriety protocols that run the temperature sensors which I was not made aware of.

Unplugging the system and rebooting (as recommended) provided a few short moments of blissful quiet before the roar of the wind turbines cranking up again interrupted my reprieve.

The exirion app linked to above works perfectly - I have my quiet working environment back. My gratitude (and serious admiration) to the developer.

-n

Thanks ishootishoot! Working great soo far!!!!!

Just running the test helped with the issue for me although the test did not actually find any faults.

It doesn't seem to be working, what could I be doing wrong?