Loud iMac grinding noise on system wake

Thank you. I would guess that I'm sometimes outside that temperature range. My central heating doesn't come on until 6.30am and I sometimes get up early and wake up the iMac before the ambient temperature has reached 10 degrees - although the noise sometimes occurs later in the day, some hours after I have turned off the supplementary heating in the office. But the temperature in the adjoining room is around 23 degrees so I doubt the office could have dropped below 10. I will now check though.

This iMac is powered up 24/7 and I can't remember getting the noise on any of my rare startups. Maybe I will try powering down tonight and see what happens in the morning.

I will experimentally change my energy saver settings too - just in case setting the computer to never sleep will cure it.

This week, outdoor temperatures when I first wake the computer have been around minus 5 degrees - and I would guess around plus 5 degrees in my office before the heating comes on.

Thanks Allan. I added some temperature information in my reply to den.thed (forgetting to say that the air is quite dry in my house so I doubt it's a humidity issue).

My optical drive is capable of making some interesting noises when it's trying to read or eject an uncooperative disc but nothing similar to this noise. Anyway, it is invariably empty. I don't know anything about fans - why would a cooling fan come on when the computer and everything else is cold?

I accept there could be dirt and dust in there; but I have opened this iMac up on a couple of occasions to swap something out and clean the inside of the screen and I gave it a good dusting each time. So the accumulation is likely 3-4 years' worth.

If nothing else works, I might have to take it apart again.

We don't often get that low here in tropical Berkshire, England but we've had a few days of it lately. I agree my iMac owes me nothing but I also run a modern Mini and MacBook Pro and Os 10.6.8 is so much better imho that I would be grief-stricken to lose it. I could buy an old Mac on eBay but you can never know what problems come with it.

So this morning, which dawned at about minus 4 degrees outside, I checked the ambient temperature in my office when I came down (about 5 minutes after the central heating system had fired up) and it was 13 degrees so it's very likely that I have sometimes woken my iMac when the temperature was below the operating spec.

Last night, I set Energy Saver to never allow the computer to sleep so only my screen was dark when I opened up. And there was no noise - presumably because no fans fired up. Nothing conclusive yet but this would be an acceptable workaround if a cure isn't practicable. I will also try powering down overnight with my previous Energy Saver settings.

What is it costing me to leave the computer awake 24/7? I assume it is consuming more power even though inactive…and also that it will hasten its EOL. But the end can't be far away anyway on an 11-year old computer.