Deep humming noise that come and goes periodically

[passatgt]

Hi!

I have a 2011 mid 21.5 inch iMac. Today i noticed a weird humming-buzzing noise that appears to be periodical. 12 second silence, 1-2 second noise. And keeps going on and on, but the interval between the noise is exactly 12 seconds. I recorded an audio file, turn up the volume and i think you can hear it clearly:

http://f.cl.ly/items/2I3i2g102s171l1w341v/buzzing.m4a

(noise starts after 1 sec)

Any idea what the hell is this? Driving me nuts...

 

[DeltaMac]

Log out, so you are at the login screen.
Do you still hear that periodic noise?

Does the noise come from the speakers? Plug in headphones to check for the same noise in your headphones.

 

[passatgt]

Just tried logging out, no luck, noise is still there. I have external speaker plugged in already and the noise is coming from somewhere else, not from the speakers and not from the internal speaker.

 

[macmee]

Is it the hard drive?

 

[DeltaMac]

I would agree that your next possibility is the hard drive. Hard drives will often make a variety of odd noises, when "death" is near.

 

[passatgt]

I have an SSD installed so i was able to unmount the factory hdd, no luck, i can still hear the noise, so its not the ssd. I'm guessing its the power supply?

 

[poematik13]

it could just be a vibration of something, try repositioning the computer and make sure there isn't a cable caught under the stand or something against the unit. (i know that sounds dumb).

 

[DeltaMac]

Be sure to try each fan. You can use a utility like SMCFancontrol to run up each fan individually. That may make the sound more apparent, if it is one of the fans - Listen carefully as each fan changes to hear if there is any change to the noise.

 

[Brian Y]

Sounds like a fan dying to me.

 

[passatgt]

Just tried with smcfancontrol. Looks like the issue is with the HDD fan. When i set the minimum rpm to a higher value and save it, it will reach 3000rpm but after 2-3 seconds it will stop completely. Thanks. Is it possible to disable the fan with software? I don't have any other issues and looks like the noise is generated when the fan fails to start spinning, so until i can get it fixed it would be a temporary solution.

 

[DeltaMac]

... Is it possible to disable the fan with software? I don't have any other issues and looks like the noise is generated when the fan fails to start spinning, so until i can get it fixed it would be a temporary solution.

I wouldn't recommend EVER disabling a cooling fan.
If that's the cause of the noise, then you should not ignore it. If you disable the fan, you are also liable to forget that the fan is disabled (out of sight, out of mind, etc.), and it's a cooling fan - it's there for a reason.
Now that you know the cause, allow the noise to annoy you until you get the fan replaced (which will now be sooner than later)

just my opinion, of course.

Something to look at: you can check that the intake for that HDD fan is completely clear. It's the round grill on the back of your iMac, directly above the power connector, and underneath the hinge for the stand. Might even be clogged. You can gently brush any lint, etc, out of that intake.
The fan is fairly low cost. http://www.amazon.com/Apple-iMac-Dr...=UTF8&qid=1414884098&sr=8-1&keywords=922-9121
However, the replacement is not easy. It requires taking virtually everything out of the iMac, even the logic board.
This video guide will show you what's involved with that: https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2428+Hard+Drive+Fan+Replacement/5953