I wouldn't just drop lubricant on the fan. If the fan is clogged with dust or nicotine it will slow it down even more.

If you can access the fan use some 91% (or 91-99%, do not use anything with a lesser percentage as they can be conductive and if accidently dropped onto a system / logic board can cause electronic components to short out) Isopropral (rubbing / the white NOT green stuff) alcohol and a brush to clean away the dust and debris then add about 2 drops of 3 and one oil down in the center where it gets inside of the fan housing.

If it still makes noise, you probably have a bearing going out - these fans spin hundreds of revolutions per second and will simply wear out ... in this case, replace it, the fans only cost a few bucks.

If you changed the hard drive to one without Apples firmware, this can cause run away fan issues.

give it a good oiling with some grease ..don't use WD40 it good but lasts only a few days ..give it a good clean as well.

Check if there are layers of dust on the blades or in the housing of the fans. Buy a can of pressured air and blow off the grit. Be careful to hold fragile components in place so they dont break, e.g. fan blades.

I have a 27" Mid 2011 2.7 i5 , It was working well , I have upgraded it to 3.4 i7 (2600) and changed the hard drive with another 1TB from my 2010 27" imac after that when it comes to osx the fan works with full speed , I've reset the SMC and used fac control software but nothing changed please help me !!!!!

I've seen this problem before and it was because the hard drive was replaced with a different brand but the hard drive sensor cable was not changed. Different brands use different temp sensor cables. That might be the problem.

there is no temp sensor on 2011 27" iMac hard drive and I've replaced it with the 1 TB WD from my 2010 iMac and I'm sure that its the cpu fan not the hard drive fan that runs at full speed , I've installed windows 8 using bootcamp and when I run fan control software in windows it shows that the GPU temperature goes to 69 to 74 degrees I think that might be the problem

if you hold alt option key and keep viewing the boot screen even for a long time the fan works just normal but when it starts to boot to an OS (windows or OSX) it starts to work at full speed

In the iMacs I have worked on there is a temp sensor cable that runs from the hard drive to the logicboard. You are right that the hard drive itself does not have a fan but if the iMacs use a specific kind of hard drive and there is a thin cable that runs from the hard drive to the logicboard that is called a hard drive temp sensor cable. If that cable is missing it will cause the fan to run at high speeds. I learned about this while working at an apple store. Look it up.

There is no a hdd temp sensor in 2011 models , there was till 2010

I had the same problem when I swapped out the HD in my imac 27" mid-2011 with an SSD drive. I believe the fan began running full throttle because it did not know how to process the temperature readings of the new drive etc.

I searched online and came across this website - http://www.hddfancontrol.com

(Parenthetically- I am not that tech savvy but I do not recall seeing or detaching a temp sensor or wire)

I installed their program and have not had any problems since (it's been at least 6 months).

Good Luck!

Mike

That is true. The software from the website above is fantastic. About $35. The iMac went completely quiet. You can set the temperature and speed of the fan as well. I strongly recommend it. Tin