(BD, meant to reply to Sheila)
I would first recommend trying an SMU Reset for this. Don't see why replacing the fans will solve the problem.
+Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.+
+Unplug all cables from the computer, including the power cord.+
+Wait 10 seconds.+
+Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.+
+Let go of the power button.+
+Press the power button once more to start up your iMac.+
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Also run the Hardware Test. It will be located on one of the discs that came with the iMac, usually the "Additional Software and Hardware Test" disc. Run it in Extended. May need several run-throughs to detect an error.
*+Using Apple Hardware Test+*
+Before using Apple Hardware Test, disconnect all external devices with the exception of keyboard, mouse, display, and speakers. You should also disconnect the Ethernet network cable. If you are using the MacBook Air, you may need to connect the MacBook Air SuperDrive. Print out these instructions before proceeding with the following steps.+
+1. Restart your computer, holding down the “D” key while the computer restarts.+
+2. When the Apple Hardware Test language chooser screen appears, select the language appropriate for your locale, and press the Return key or click on the right arrow button.+
+3. If your computer is not supported by this version of Apple Hardware Test, an error dialog will be displayed. You should Shut Down the computer, locate the correct DVD containing Apple Hardware Test for this computer, and repeat the above steps.+
+4. If your computer does not restart to Apple Hardware Test:+
+a) Restart your computer with the DVD containing Apple Hardware Test. This should be the same DVD this document resides on. Hold down the “D” key while the computer restarts.+
+b) If your Mac has a Firmware Password enabled you need to disable it first using the Firmware Password Utility when booted from the Mac OS X installation DVD.+
+5. When the Apple Hardware Test main screen appears, follow the onscreen instructions.+
*+Advice about using Apple Hardware Test+*
+If Apple Hardware Test does not detect a hardware failure, the problem may be software related.+
+If Apple Hardware Test detects a problem, it displays an error code. Make a note of the error code before pursuing support options.+
+Alternatively if you are able to startup your computer from Mac OS X, you will be able to retrieve the error code, and optionally send this information to Apple, using the System Profiler utility application (which can be launched from the "About This Mac" dialog).+
+If your computer has more than the minimum configuration of RAM, the Extended Test may take significantly more time than indicated on the screen. Your computer can use memory modules or DIMMs even when the Power On Self-Test, which runs when the machine is turned on, detects part of a module as being bad. When you start up your computer, Apple Hardware Test reports failed memory modules in the Hardware Tests panel.+
+On computers with intelligent thermal management, it is normal for the fans to be running at high speed while using Apple Hardware Test.+
Message was edited by: WZZZ
