Introduction
I was surprised to find that the top right fan on the G5 iMac (first generation) changed design; one had 36 blades and gave an annoying whine, while the other design has 29 (a prime number) and yields a comforting hum.
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            To the left is the desirable design and to the right the one that generates more noise. 
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            ifixit carries both designs and was most helpful in ensuring that I got the correct one. 
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            Another way to tell the difference is that the bad design is attached with cross head screws, while the good design is attached with torx screws; however the old screws can be used on a new fan. 
 
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                            Conclusion
                          
                          The better fan turns an almost unusable G5 iMac into one that is much more acceptable.