It could be if the computer is getting hot from use. This will cause the fans to ramp up in order to cool it down. You may want to check for something running amok and causing the computer to heat up. I would also install a utility to monitor temps and fan speeds: iStat Menus 4.21.
Open Activity Monitor in the Utilities folder. Select All Processes from the Processes dropdown menu. Click twice on the CPU% column header to display in descending order. If you find a process using a large amount of CPU time (>=70,) then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that helps. Be sure to note the name of the runaway process so you can track down the cause of the problem.
Mavericks and later
Open Activity Monitor in the Utilities folder. Select All Processes from the View menu. Click on the CPU tab in the toolbar. Click twice on the CPU% column header to display in descending order. If you find a process using a large amount of CPU time (>=70,) then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that helps. Be sure to note the name of the runaway process so you can track down the cause of the problem.
So I installed iStat Menus. What I can see is that when I hear the fan noise the fan speed goes up to 4000 RPM. When the noise stops the speed goes down to 1400 RPM.
I can also see that there is no clear correlation between sensors temperature and fan speed. Meaning, when the fan goes from 1400 to 4000 RPM the sensors show the same (more or less) temperature.
Most of the CPU is running at very low load (most of the time about 2% load). Again, no correlation between CPU load and fan noise / speed.
One other thing I noticed is that sometime the fan noise appears when the computer is supposed to be sleeping.
Is that normal iMac behavior?
Did you ever resolve this issue Pashty ?
I am having a very similar issue with my late 2013 iMac 21.5.
The fans have some specific behaviors;
- Gung ho mode: Fans run at full speed
- Burst cycle mode: The fans will typically have a 2-3 second burst at full speed every 15 seconds or so.
- Normal mode: The fans run the way you expect them to, and all is well.
The problems started a couple of months ago. The iMac fans would on occasion go into gung ho mode for several minutes. Typically I would restart the iMac, and the fans returned to normal. However, in the last few weeks, the fans go off more often than before and restarting does not help. Additionally, the iMac has started going into the Burst cycle mode several times a day. (sometimes only seconds after turning the iMac on) .This will often go on for 20-40 minutes before the fans either settle down, or go into gung ho mode. Even while sleeping, the fans sometimes go gung ho or burst cycle.
I have downloaded SSD Fan Control and MenuMeters. During the latest gung ho, SSD Fan Control showed the fan running at 4000rpm. MenuMeters showed all temperatures were normal, and Activity Monitor showed no process with more than 2-3% CPU usage. I have even tried resetting the SMC by unplugging for several minutes.
I bought the iMac in October 2014. This cannot be normal behavior ?
I have a late 2009 27inch imac and have the same problem...started about 2-3 years ago. Try resetting the SMC...sometimes it works for a few days...then the fan kicks back on again....always at full speed. I have nothing running...just does it at startup....other times will wait until I've been using the computer for a while...then kicks in full speed. Very very frustrating that there has been no solution that completely solves the problem...resetting the SMC only seems to be a short term fix as this keeps happening.
Any other solutions?