sounds like a lot of disk activity, but ...

Doug Lassiter · 11-25-2012, 02:04 PM

Intel Core Duo iMac running 10.6.8. Original Seagate 500GB HD.

Just started making faint noises that sound a LOT like continuous disk activity. The sound seems to be coming from the lower left of the screen. Isn't that where the HD is?

Now, the funny thing is that there is no other evidence of such activity. No process is taking a lot of CPU%, the activity monitor shows occasional bursts of disk activity, but nothing that correlates with the nearly continuous sound. HD verifies, and checks out in SMART. In fact, I kill off most of the processes shown by activity monitor, and the noise is still there. Nothing funny in Console messages.

File Vault is off, as is Time Machine.

Same noise in different accounts and even in the login window. The noise DOES stop when the machine goes into sleep mode, though.

I guess I shouldn't be worried about it except, um, it didn't do that before. It's not anywhere near annoyingly loud, but a bit of a distraction in a very quiet room, sitting right next to it.

Any ideas?

DeltaMac · 11-25-2012, 05:35 PM

The noise depends on if you have a 17-inch (HD in top center) or 20-inch (HD left of center)
Lower left will more likely be one of the fans.
Your fans run any time power is on, so could be a fan starting to fail, or just needs a good cleaning. You would have a nice challenge just to clean (or even replace) just one fan.
You have 3 fans inside. If you run one of the utilities that can control the fans, you can increase the speed on the individual fans, and quickly find out if that's the problem.
(The fans don't run in sleep mode)
Here's a good one to help with that: http://www.macupdate.com/app/mac/23049/smcfancontrol

Doug Lassiter · 11-25-2012, 05:58 PM

Thank you. Very informative. Right after I posted I was thinking "Um, fan?" That must be it. The giveaway, as you say, is that it makes the same noise doing anything except sleep. As to cleaning, I could just try the vacuum-cleaner-over-the-vents-trick, and hope for the best.

I used smcfancontrol to exercise the three fans. The CPU fan sounds like it's coming from where I heard the sound, at lower left. BUT, when I jog that fan over a range of speeds, I can't reproduce the sound I was hearing. That fan actually sounds really smooth.

DeltaMac · 11-25-2012, 06:29 PM

I think it's more likely a fan.

Here's one method to help eliminate the hard drive as the cause of the noise:
Boot to your installer DVD.
Run Disk Utility from the Utilities menu.
Select your system's hard drive, then click the Unmount icon in the disk utility's toolbar.
The hard drive should stop running after a few seconds. The optical drive may continue to run for another few seconds (still making some noise), so give it about 30 seconds, and note if your noise continues. If it's a fan, the noise should not change very much.
If that's it, good luck on sucking dirt from the outside. The CPU fan is a couple of inches from the bottom vent. I think it's not likely that cleaning the bottom vent will do much to help - but, then it won't hurt anything to try.

Doug Lassiter · 11-26-2012, 09:45 AM

Actually, I don't hear the noise anymore. You don't suppose that in revving up the CPU fan, I cleared out some debris in it? I'll see if it comes back.

DeltaMac · 11-26-2012, 11:02 AM

Yes, that's possible that you moved some 'dust bunnies' around. That fan blows "up" through the CPU heat sink. So, possibly, you also succeeded in blocking off some of the cooling passages. That may not be the case, but still possible.
It's a good reason to leave the SMCFan Control to monitor the various internal temps, and allows you to manually control the fans if you note some areas (like the CPU) getting warmer than usual.
It might be a good plan to get inside to clean the various cooling passages more completely - but that's quite a job on the older intel iMacs. Monitoring the temps may help you decide when that might be necessary.

Doug Lassiter · 11-26-2012, 04:40 PM

Is there a listing of recommended temperatures to expect from these three sensors? That is, if fan isn't working well, it would be nice to know when a line is being crossed, temperature-wise. If I hear a noise from an otherwise quiet fan, I might wonder if it is doing the colling it should be doing.

smcFan Control is a little opaque, in that I'm not sure what's going on with it. I gather I can command max and min speeds for the fan, but the fans are always running, at least at the minimum speed. Is that the case? I don't hear anything. It is a bit odd that the menubar display doesn't say what fan or temperature I'm looking at. I have to dig down and see what prefs are set.

DeltaMac · 11-26-2012, 05:40 PM

The "recommended" temps would range all over the place, but in my view, 75-80 C or more than 170 F would ordinarily cause the fans to kick up. The average temp will vary, depending to some extent on your ambient temp at the computer's location, and the mix of software that you are running at that time.
If the processors come under heavy load, you might even see temps spike over 100 C! I would say that you don't want temps that high for more than 30 seconds or so. The fans should automatically be at full speed at those kind of temps.
So, monitor your temps for a week or so, under different conditions/software - whatever you normally do. You'll soon know when the temps are going out of your normal range.
I use iStat Pro, which monitors each fan speed and temps for the various sensors in the Mac. It doesn't directly control the fans, but gives more detail about temps to performance in a Dashboard widget.
Yes, the fans are always running. You usually don't hear the fans at all, unless they kick up to higher speed.

11-25-2012, 02:04 PM	#1
Doug Lassiter Major Leaguer Join Date: Apr 2004 Location: Burnett, Texas Posts: 289	sounds like a lot of disk activity, but ... Intel Core Duo iMac running 10.6.8. Original Seagate 500GB HD. Just started making faint noises that sound a LOT like continuous disk activity. The sound seems to be coming from the lower left of the screen. Isn't that where the HD is? Now, the funny thing is that there is no other evidence of such activity. No process is taking a lot of CPU%, the activity monitor shows occasional bursts of disk activity, but nothing that correlates with the nearly continuous sound. HD verifies, and checks out in SMART. In fact, I kill off most of the processes shown by activity monitor, and the noise is still there. Nothing funny in Console messages. File Vault is off, as is Time Machine. Same noise in different accounts and even in the login window. The noise DOES stop when the machine goes into sleep mode, though. I guess I shouldn't be worried about it except, um, it didn't do that before. It's not anywhere near annoyingly loud, but a bit of a distraction in a very quiet room, sitting right next to it. Any ideas?

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11-25-2012, 05:35 PM	#2
DeltaMac League Commissioner Join Date: Jan 2002 Posts: 7,996	The noise depends on if you have a 17-inch (HD in top center) or 20-inch (HD left of center) Lower left will more likely be one of the fans. Your fans run any time power is on, so could be a fan starting to fail, or just needs a good cleaning. You would have a nice challenge just to clean (or even replace) just one fan. You have 3 fans inside. If you run one of the utilities that can control the fans, you can increase the speed on the individual fans, and quickly find out if that's the problem. (The fans don't run in sleep mode) Here's a good one to help with that: http://www.macupdate.com/app/mac/23049/smcfancontrol

11-25-2012, 05:58 PM	#3
Doug Lassiter Major Leaguer Join Date: Apr 2004 Location: Burnett, Texas Posts: 289	Thank you. Very informative. Right after I posted I was thinking "Um, fan?" That must be it. The giveaway, as you say, is that it makes the same noise doing anything except sleep. As to cleaning, I could just try the vacuum-cleaner-over-the-vents-trick, and hope for the best. I used smcfancontrol to exercise the three fans. The CPU fan sounds like it's coming from where I heard the sound, at lower left. BUT, when I jog that fan over a range of speeds, I can't reproduce the sound I was hearing. That fan actually sounds really smooth.

11-25-2012, 06:29 PM	#4
DeltaMac League Commissioner Join Date: Jan 2002 Posts: 7,996	I think it's more likely a fan. Here's one method to help eliminate the hard drive as the cause of the noise: Boot to your installer DVD. Run Disk Utility from the Utilities menu. Select your system's hard drive, then click the Unmount icon in the disk utility's toolbar. The hard drive should stop running after a few seconds. The optical drive may continue to run for another few seconds (still making some noise), so give it about 30 seconds, and note if your noise continues. If it's a fan, the noise should not change very much. If that's it, good luck on sucking dirt from the outside. The CPU fan is a couple of inches from the bottom vent. I think it's not likely that cleaning the bottom vent will do much to help - but, then it won't hurt anything to try.

11-26-2012, 09:45 AM	#5
Doug Lassiter Major Leaguer Join Date: Apr 2004 Location: Burnett, Texas Posts: 289	Actually, I don't hear the noise anymore. You don't suppose that in revving up the CPU fan, I cleared out some debris in it? I'll see if it comes back.

11-26-2012, 11:02 AM	#6
DeltaMac League Commissioner Join Date: Jan 2002 Posts: 7,996	Yes, that's possible that you moved some 'dust bunnies' around. That fan blows "up" through the CPU heat sink. So, possibly, you also succeeded in blocking off some of the cooling passages. That may not be the case, but still possible. It's a good reason to leave the SMCFan Control to monitor the various internal temps, and allows you to manually control the fans if you note some areas (like the CPU) getting warmer than usual. It might be a good plan to get inside to clean the various cooling passages more completely - but that's quite a job on the older intel iMacs. Monitoring the temps may help you decide when that might be necessary.

11-26-2012, 04:40 PM	#7
Doug Lassiter Major Leaguer Join Date: Apr 2004 Location: Burnett, Texas Posts: 289	Is there a listing of recommended temperatures to expect from these three sensors? That is, if fan isn't working well, it would be nice to know when a line is being crossed, temperature-wise. If I hear a noise from an otherwise quiet fan, I might wonder if it is doing the colling it should be doing. smcFan Control is a little opaque, in that I'm not sure what's going on with it. I gather I can command max and min speeds for the fan, but the fans are always running, at least at the minimum speed. Is that the case? I don't hear anything. It is a bit odd that the menubar display doesn't say what fan or temperature I'm looking at. I have to dig down and see what prefs are set.

11-26-2012, 05:40 PM	#8
DeltaMac League Commissioner Join Date: Jan 2002 Posts: 7,996	The "recommended" temps would range all over the place, but in my view, 75-80 C or more than 170 F would ordinarily cause the fans to kick up. The average temp will vary, depending to some extent on your ambient temp at the computer's location, and the mix of software that you are running at that time. If the processors come under heavy load, you might even see temps spike over 100 C! I would say that you don't want temps that high for more than 30 seconds or so. The fans should automatically be at full speed at those kind of temps. So, monitor your temps for a week or so, under different conditions/software - whatever you normally do. You'll soon know when the temps are going out of your normal range. I use iStat Pro, which monitors each fan speed and temps for the various sensors in the Mac. It doesn't directly control the fans, but gives more detail about temps to performance in a Dashboard widget. Yes, the fans are always running. You usually don't hear the fans at all, unless they kick up to higher speed.