Since I have intalled Mac OS 9.0.4 update on my iMac 400 DV, my computer is much more noissy than before. I have understand that in this new update , Apple has made changes on the "power management" . What is it about ? I can ear the spin of the internal HD all day.
What can I do to stop that ?
Help.....

I have the same problem, except that my iMac started doing it 3 days after I got it. So I immediately brought it in. They replaced the hard drive because it was
"A little louder than normal". After I got it back it seemed ok, but then that same day it started doing it again. The software version that I had installed was 9.0 and even with upgrading the noise still remains. Pretty annoying isn't it?

I updated to 9.0.4 (iMac DV SE); no change in noise level. maybe your HD decided to have a problem by coincidence?