Tip: if you get a high pitched whine from your iMac, try this

[Mac32]

Hi,

If you notice high pitched whine, and it goes away when doing specific tasks (ie. the whine is only there when CPU is idle), you can do this:

To remedy this:
Use command prompt, run the following command:
reg add HKLM\System\CurrentControlSet\Control\Processor /v Capabilities /t REG_DWORD /d 0x0007e066
Restart the computer.

If you want to reverse this registry change:
At a command prompt, run the following command:
reg delete HKLM\System\CurrentControlSet\Control\Processor /v Capabilities /f
Restart the computer.

This fix removes the idle CPU states C2/C3. NB: This will make the CPU a few degrees hotter during load, esp. during gaming.