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I have an iMac. Since yesterday, it has been making sounds where it seems like a hard drive is starting up then shutting down immediately. However, that can't be possible of course because the computer technically works fine—the sound occurs while I'm using the computer, and it doesn't lock up or anything like that. (Also, I thought it was an external HD but I put my ear to the back of the iMac and it is definitely coming from there.)

This started happening around the same time that I started watching high definition videos (720p h264) of a TV series. I had watched maybe ten episodes before the sounds started occurring. That would be perhaps ten hours over a 20 hour span. Also, the computer was up (hadn't shut down) for 72 hours straight.

I'm thinking it could be a fan problem perhaps, or a power supply problem?

These sounds have never happened before, even when doing stuff like playing high performance computer games.

I have also noticed that there is a high-pitched buzzing sound that occurs when this startup/shutdown sound happens. Also, the startup/shutdown sound seems to happen consistently when I start up my computer, so perhaps it IS a fan or power supply related problem?

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I would recommend taking your Mac into an Apple Store or an Apple Authorized Service Provider for service. To be sure what the problem is, you really need to take the machine apart. Sounds like a failing fan to me, though. –  Nathan Jul 24 '12 at 2:45
    
Okay thanks. My concern is that the problem isn't consistent enough to bring it in to the Apple Store just yet. I still have about a year on AppleCare so I think it may be better to wait a few days or weeks until the problem gets worse. Or does that sound unreasonable? Because, the last few times I saw the Apple Store, they really tried their best to avoid taking in my iMac for repairs if possible. –  Gary Jul 24 '12 at 3:20
    
I also just noticed that my computer has started locking up. I've never had that happen before. iTunes stopped playing twice and Firefox froze momentarily during one of those times. I don't know if that's related but I feel that it is because this has never happened before, and those were the only two applications that I had running at the time. –  Gary Jul 24 '12 at 3:37
    
So this the noise you hear when starting up the iMac, eg the fans are spinning up like they would when first powering on the iMac and then going quiet then randomly repeat? –  MrDaniel Jul 24 '12 at 16:49
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1 Answer

up vote 0 down vote accepted

Open it up, check for dust, reinstall the Mac OS X (fresh installation), Check whether the fan is functioning properly. If all these doesn't solve the issue, then probably you will need to get it examined by an Apple service guy.

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Opening the iMac may void your guarantee so you may be better off to contact Apple Care directly. –  patrix Jul 24 '12 at 12:38
    
yeah, @patrix is right, my suggestion is only if you are out of your warranty period. –  iGameRam Jul 24 '12 at 14:24
    
So it worked? I am really happy i could help :-) –  iGameRam Jul 31 '12 at 5:40
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