[rhnes]

A few months back, a fan in my iMac G5 randomly started running at full speed. It will be quiet for a few seconds, rattle a bit, rev up a little like a car, and then kick in at full speed. After running for however long it feels like, it will randomly stop, only to repeat the process again a minute later. It sounds like a vacuum cleaner.

As odd as it seems, at first, I could get the noise to stop for a few minutes (probably 30 at the most) by tilting the screen forward or backward a bit whenever it started. Within the past few weeks this has stopped working altogether though. It's almost constantly making noise.

I'm planning to take it in to an Apple Store as soon as I have the time, but I'm currently trying to get some audio work done, and the noise is causing tons of problems. I'm hoping someone can offer advice on something to try in the meantime, or at least tell me what it seems like the problem might be?

20" iMac G5 (iSight)
2.1 GHz PPC
10.4.11

When the fan is at it's loudest, iStat Pro reports:

Temps (°C)

CPU - 58
GPU - 43
Incoming Air - 45
HD Temp - 52
GPU Ambient - 56
Optical Drive - 38
Mem Controller - 85
HD: Macintosh HD - 55

Fans

CPU - 1368rpm
Hard Drive - 2020rpm
Optical Drive - 1567rpm

Is any of that out of the ordinary?

The vent at the back of the computer seems to be the only place that has a lot of air blowing out. Nothing from the vents on the bottom.

I've search around the internet a bit, and I've tried a few things that I've seen suggested. I've reset the PRAM, NVRAM, and SMU. I've tried repairing disk permissions. I've unplugged the computer for extended periods of time. I've done a fresh install of Tiger.

Is there anything I'm missing? Any guesses at what might be wrong? Any help is appreciated. I've quite a bit of work to do, and the computer is almost unusable.

Thanks.

[iMan G5]

The only main differences between your stats and mine are that your processor is a lot cooler for some reason. Your HD fan is a little faster too.
Here are mine as of the moment. BTW, I have the 17 inch 1.9Ghz model.
In Celsius, just woke from sleep all night.
CPU - 66
GPU - 72
Incoming Air - 28
HD Temp - 39
GPU Ambient - 66
Optical Drive - 35
Mem Controller - 78
HD: Macintosh HD - 34

Fans:
CPU:1650's rpm
Optical Drive:1350's rpm
Hard drive:1480's rpm
Thats with about 40-50% CPU usage. Which is Web browsing with FF, Xbox friend list app, Adium, iTunes, Stickies, and Gyazmail. If you had someone work on it they must have cleaned it out. These things are pretty hard to open. I have wanted to open mine and clean it but it needs some type of tools I don't have.

[P]

Incoming air at 45 C? That's quite hot. Is it really 45 degrees in your room?

The vents at the bottom are intake vents. As I suggest whenever anyone asks this: Turn of the Mac and vacuum those vents to make sure that they're not filled with dust. Dust in those vents will block airflow and force the fans to work harder.

[rhnes]

Thanks for the replies.

I hadn't really noticed the incoming air temperature. It's down to 35-37 now, but that's still a bit hotter than the 25 that it actually is in my room. I've cleaned out the vents a few times since this all started, but I'll definitely give it another try. Is the incoming air temperature alone enough to cause this much noise?

My hard drive fan speed and temperatures are also notably higher than iMan G5's. The odd thing is that they don't seem that much higher, and iStat Pro doesn't say the fans are running any faster when the computer is at its noisiest than when it's completely silent. The hard drive fan is sitting at around 1680rpm, at the moment.

Still, I'm a little concerned now that it might be the hard drive causing the problem. If the hard drive was failing though, wouldn't I have seen the effects of that by now? It's been a few months, and there are no notable performance problems.

Are there any fans that iStat Pro isn't mentioning that might be the culprit? Any ideas as to what might be the problem, or suggestions on what might fix it?

Thanks.

[P]

Newton's law of cooling (which is what you get for convection cooling, ie fans blowing on something) states that the energy transferred equals h(T(hot)-T(cool)). The energy transferred is constant, for a given component at a given workload. h is determined by medium properties (essentially, air composition) but also the flow rate. If T(cool) - the temperature of the air used to cool your equipment - goes up, h has to go up as well to keep T(hot) within reasonable limits.

If you check, you see that the difference in incoming air temp and HD temp is 9 degrees for iMan G5, and 7 degrees for you. This means that your fans have to work a little harder to keep the HD cool, and yet they don't quite succeed. My guess is that the Mac is trying to keep the HD below 50 C, if possible, and it has problems doing that when the incoming air is as hot as 45 C. It's boosting fan speed as a result.