On March 13th I had another 2009 iMac owner write his system (w/base GT120 GPU) is being returned (due to the fans running at high speed constantly):

(added 3/13/2009)
"I just read your article (this page/post below from yesterday). I have huge problems with my new (2009) iMac's fan since day one. But my problem does not have anything to do with heavy gaming - the fan is on (loud) every time I'm using my new iMac. When I start the iMac the fan turns on (loud) and it never turns off before shutting the Mac down.
That's really annoying. (The fans run all the time normally but at a lower rpm and not noticeable usually when the system isn't heavily stressed/running at higher temperatures.)
I made a complaint to Apple and they're sending me a new iMac to swap with the defective one I received a week ago.

My iMac specs:
24" iMac 2009
2.93GHz Intel Core 2 Duo
4GB RAM
640 GB Hard drive
NVIDIA GeForce GT 120

(I asked if Apple had him do any troubleshooting on the iMac (i.e. reset SMC, etc. - ref: the notes in this 2009 Apple doc iMac: Fans run at full speed after computer turns on that suggests resetting the SMC) and if he had installed any 3rd party addons, migrated anything over from an older mac, etc. (He later said resetting SMC did not help.) If the fans run all the time and SMC reset does not help, run Apple hardware test (on the supplied restore disc #2/Apps disc) as anm earlier report here noted AHT indicated an error code related to the Logic board (Sensor related). Apple is replacing his iMac now.)

No, they didn't ask me to do anything. They just quickly said it's a mistake and that they will send an alternate device. I didn't install any addons, just copied my personal files from my MacBook Pro.
Regards, Stefan"

I asked Stefan to run Apple hardware test to see if it reported any "SNS" (sensor) related errors - a followup from another 2009 iMac owner with high speed fans said AHT showed a "4SNS/1/40000000:T00P-89.875" error. (SNS errors reportedly are sensor related.)

(The following was moved off the March 12, 2009 news page due to length.)

Posting as a FYI as I had a couple emails last week from readers considering a new iMac that had questions/concerns about heat/fan noise, etc. with the higher end GPU options. (CPU can also be a factor, as well as usage/load of course.) Tolerance for fan noise varies - some people don't mind it, others hate any fan noise. (I'm used to it on Towers and PCs at least. However the original (first model PS/case fan) MDD tower's noise (frequency) was incredibly irritating.)

(3/12/2009 - updated 3/24/2009 w/comments on Apple hardware test error code (logicboard related))
"New (2009) iMac Fan Noise
I received my new early 2009 iMac yesterday (24inch, 3.06GHz w/GeForce 130 GPU). It's great except for the really loud fan noise. When I sit down to play world of warcraft or any other game (see updated comments below on WoW vs Prey Demo) the fan kicks up to full speed within 5 mins of play time. It's really loud and annoying. None of my other intel-based iMac computers do this.
-Jeremy"

I'd expect some fan noise (increase in speeds) during gaming (which stresses the GPU and CPU) but not sure how loud this is. (If it's really noisy maybe it's just a bad fan/assembly issue - I've not heard one of these in person.) I also asked if he could try some non-game stress tests (for instance converting a large video to H.264 using Handbrake (free), a large iMovie export, etc.) just to see if the GPU was a primary factor.
Update: Here's a later reply to earlier questions: (Updated comments again on Fri. Mar. 13th)

"It has to be GPU related as I converted the 1966 Batman movie to h.264 using handbrake while messing around in parallels 4.0 running Windows XP Pro while listening to itunes with no speed change of the system fans.

The fans were running at the following speeds during handbrake conversion, this is pretty much the same as while idle.

• CPU 1200 rpm
• HDD 1600 rpm
• ODD 800 rpm

While Playing World of Warcraft for a while the fans climb up as high as 4800 rpm and are extremely loud. And stay that way until I turn the iMac off and let it cool back down to an acceptable temp.

The GPU usually idles at around 52-55 degrees C, while Playing WoW it can get up into the 65-67 range then it goes back down to 58-57 before the fan kicks into 'high gear'. (I've seen much higher temps on GPUs before.) The GPU cools down to about 45 degrees C with the fans running.

The fans run at the following speeds while in 'high gear'

• CPU 3600 rpm
• HDD 1600 rpm
• ODD 4800 rpm

(Is there a CD/DVD in the Optical Drive? Just wonder if not having a disc in the optical drive helps as the highest fan speed is the ODD and wonder if that's the main source of the fan noise.)
No disc in drive.

I wouldn't mind the fan coming on so loud if it slowed back down when the system cooled to a normal level. (So even after quitting a game and the system idle for 5-10 minutes, the fans are still running full bore? Have you tried an SMC reset?)
The fan does not spin down until I shut down the iMac and leave it off to cool down for about 5-10 mins. Putting it to sleep for 5-10 mins works too. The fan then spins back to normal speed. When the computer is powered on and even when the temps are within normal ranges the fan will not slow down once they get going.

I downloaded the DEMO of prey (Doom 3 engine based) and played it for about a half an hour and the fans did not budge from their normal speeds. I guess WoW pushes a GPU pretty hard. Neither of the two needed a disc in the drive to run.
(Odd that Prey didn't spin up the fans... with HQ settings I'd expect it to tax the GPU well - one difference I can think of with WoW is Network I/O.)

I tried resetting the SMC two different times with no luck.

I posted my problem to Apple's support forums and about the best the users could offer is that my iMac is defective, which I highly doubt. My thoughts are that things like this have happened in the past (G5 fan issues come to mind) and apple fixed them with a firmware update. I'm hoping this is the case.

(Update: On March 24th, 2009 he wrote)
"I have an update to my saga of the 2009 iMac with loud fans. I called Apple's Support Line (SOS APPLE) and talked to a rep, we concluded there was a hardware problem (unknown what type). He recommended I take it to a service center to have it looked at and repaired.

That day (monday 3-16-2009) I took it in to our local Apple "Service Center". A week passed and I called to see if it was finished and it was. When I picked it up they said there was nothing wrong and was told "that's just how apple products run". I was dumbfounded, especially after Apple said there was a hardware problem.

I took my iMac home and plugged it in. Now the fans ran ALL the time. (Someone else had this problem - I sent info/links on resetting SMC (apple doc suggests that) but he replied it didn't help.) Not sure why, but the fans didn't kick off at any point. (Apple's doc I linked to earlier here (see page above) says it can happen if the power button is depressed while plugging in AC power for instance, but it could also be a sensor problem (hardware or software) - fans default to max RPM (a fail-safe mode) if not controlled by the OS's thermal management.) I figured that the hardware problem had gotten worse. This morning I went to my original packaging and got my restore disc #2 (applications disc) to run the hardware diagnostics. I ran the diag disc and it found a hardware error (4SNS/1/40000000:T00P-89.875 - if that means anything to us normal people).
(FYI - A reader replied that error is logic board related. I asked for more detailed info - i.e. what component failed the test - I'm guessing a sensor? (used for fan control). I was later told SNS errors are sensor related.)
I'm not sure how such a easy to replicate error was over looked at the "service center".

Anyway, I called Apple back about an hour ago and the are sending me a new computer. Problem solved (I hope). The technician on the phone said that they definitely want problem computers like this one back because their engineers dissect them to find out if these problems affect the product line.
cheers, jeremy"

Hopefully these are just random failures and not related to a suspect batch (component issue, etc.) of logic boards. Although it's interesting Apple recently (Mar 4, 2009) updated their doc on troubleshooting iMac fans running all the time.
(My reply to his original mail that mentioned G5 fan noise) My first-model (2003) Dual 2GHz G5 had one noisy Power Supply fan (of the two inside) right out of the box. The Power Supply was replaced (took 2 tries, they originally sent the (lower rated) PS for single CPU G5 tower, but I spotted the difference before they tried to install it). And a few times over the years after an OS X update the fans ran abnormally high but the old tip of toggling the ES performance option solved that. Apple of course has done firmware updates (SMC and EFI) in the past to some models (bug fixes/improvements) and I still remember past posts (and pix) here of the first MacBook Pros with thick slabs of thermal compound on the heatsink/heatpipe to GPU/CPU mating surfaces. (And as always, a new model may have some "first rev" issues.)

(I had written to an ATI contact about wattage/specs of the iMac's ATI 4850 (mobile GPU I was told) vs the Nvidia GT130. He said he'd get back to me, but have not gotten any details.)
(This topic also reminded me of an old friend/hardcore PC gamer (RIP) that used to say "If you hear the fans when playing, you don't have the speakers loud enough".)