21" Imac Fan Loud and Random

Question

Hi there,
After trawling these forums and others, my problem has driven me to the point where I thought id post my own thread on here.
I have had my 21" Imac for about 5 or 6 months. Up until about a month ago it has worked perfectly, until one day, whilst Idle, a fan inside kicked in randomly. Ill detail the circumstances and information here:
Fan, which is blowing air out of the top slit on the left hand side of the imac has tendancy to turn on randomly
When this happens it is VERY loud (for a computer fan)
When this happens it continues until I put the imac to sleep, or shutdown/restart, which buys me a bit of time
I have checked temprature readings and highest at these times have been about 60-70 deg celcius at the GPU, nothing ever higher than that
Ambient temp never been more than about 25/26.
This happens whether idle, or if im using a relatively heavy application all the same. Most of the time when it kicks in ive checked the CPU usage, which has been very low (10% or so) with just a few chrome tabs open.
I have tried reseting the SMC/SMU many times (switching off, unplug for over 15 secs, switch on) (and method where you hold power button for 5 secs before you plug it back in) The fact that this has not worked is very frustrating as most of the other problems on the forums seem to be resolved by this.
I have reset the PRAM twice.
I ran the imac hardware diagnostics (hold D at startup) last night, one quick test and one extended test, showed no problems.
The funny thing is that i experienced the same problem during the test. I am sure it was not part of the test as it continued well after the duration of it.
This problem has really started to get on my nerves now, as the Imac is a core part of Sound engineering and music production related work and study - as you can imagine the noise is a real bane of this.
Any help will be greatly appreciated guys, cheers.
Thanks in advance.
SK
System -
21.5-inch, Mid 2011
Processor  2.5 GHz Intel Core i5
Graphics  AMD Radeon HD 6750M 512 MB
Software  Mac OS X Lion 10.7.2 (11C74)

Answer 1

Hi Vance
I feel your pain mate. However, having said that, here is the situation since my last post-
The problem persisted all the way up to early september when i left home for uni.
Since, it hasnt reoccured once. There are two reasons I can think of for this:
A) packaging up the imac and moving it a few hundred miles in the car may have knocked something (e.g. A loose sensor connection) back into place. I can imagine that this would therefore mean the problem may appear in the future, when relocated again.
B) i have a basement room at uni which provides a very stable, relatively cold temptature (thank god with no damp, an ideal hideout) at home the room used to get ALOT hotter (thick carpet, and more heat from larger studio monitors/subwoofer etc) worth saying though this would still happen when cold at home. Maybe it is environment related?
Maybe also worth mentioning my old laptop always used to completely crash out on the basis of the graphics card overheating whilst at home, but ever few weeks where i would live elsewhere for a while it would always be fine. Very odd.
Thats probably the most i can suggest, and do let me know if you find a solution or supernatural fix to this problem. This drove me bloody nuts for ages and no forum threads on the net directly tackled this particular circumstance as far as i am aware.
Im sure anybody else in our boots would much appreciate seeing this thread rather than be corroded by the problem for so long whilst taking thier kit to the apple store isnt convenient.
Regards,
SK