Why is the fan so loud?

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

westly197 - Well, Yes & No here ...

From the '11 model forward Apple uses custom firmware within the HD to allow the internal thermal sensor (note: not all drives have this sensor) to be used as part of its SMC services which in turn controls the RPM's of the fan or fans of the system. If you try using an older non-compliant drive the fans will ramp up as SMC services will think the sensor within the HD is defective. Over time the fans will wear out and the system will be very noisy compared to what it should sound like. Heres a collection of good write-ups from OWC that describes the issue: Apple Further Restricts Upgrade Options on New iMacs and Apple’s iMac 2011 Model Hard Drive ‘Restrictions’ and iMac Hard Drive Upgradeability Expanded!

OWC created a special cable so people could put in noncompliant HD's into the '11 system '11 Upgrade kit at the same time Apple was also working with the JEDEC standards group which over see's the SMART HD standard. While it took quite a bit of time Apple was able to get the group to alter the standard so the thermal sensor with in the HD could be accessed in a common method and not interfere with the performance the the drive (which was why Apple needed to go with custom firmware at the beginning). Now HD's that comply with the updated standard no longer need the OWC cable and have the newer SMART services within their firmware.

If you look at the HD's package you should see an Apple logo on it which is the only clue that I know of. The reason you don't see anything other than that is the drive vendors still have a lot of noncompliant drives out there and they have also being going though a bit of a market collapse and contraction (being more focused on survival). I also suspect Apple may also be asking for a bit of money for the drives they certify as workable.

In the end you need to be smart shopper on what you buy.

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.

there is no temp sensor cable 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.

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

There is a cable coming from the motherboard to the back of the screen that splits in two to the left of the ribbon cable, I guess the ambiance thermal monitor is there, it is cover with some tape, if the top plug unplugs the fan goes full speed it is very easy to do by tilting the screen ,and not notice it, remove the tape very careful end re-plug the top plug and problem is solved.

Thank you Dan. I was going to purchase Apple brand harddrive used in 2013 Imacs. And then, I was going to install this hard drive in my 2011 Imac as a remedy. The harddrive you suggest, does it have to be a SSHD? Can I use regular SATA3 harddrive, as well?

I have this issue on my 27 inch 2011 iMac. First showed up after a trip overseas and it was well packed in an airport "rock band" style case. Apple store in Spain could not resolve it.

https://support.apple.com/en-gb/HT201295

Reset SMC. Worked for me on iMac 2010.https://support.apple.com/en-gb/HT201295

It is probably just clogged with dust. Have someone clean out the dust or do it yourself. disassemble it and vacuum the dust out. Make sure to clean out all of the fans and give everything a good brush and vacuum.

The thermal paste applied to the processor (CPU) may be dried out. Open up the device, then remove the motherboard. Then remove the heat sink over the cpu and apply some thermal paste.

Thermal paste at Ifixit: Thermal paste @ ifixit

Thermal paste at Amazon: Arctic MX-6

Thermal paste at Micro Center: Noctua thermal paste

After you do these, install the app called Macs Fan Control which you can download at this link. You can control your computer’s fans with this app. If you don’t mind the noise, turn them up all the way, or a better option, put the fans at 30-50% speed for optimal cooling and lower noise.

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.

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.