My G4 also makes a racket these days. I noticed this increase in noise three months ago after having a new hard disc installed (I upgraded to a bigger disc). However, I am told that a noisy G4 can be due to the build up of dust on one or more fans.
The sleep issue is annoying. I am tempted to do a clean install and see what happens.
I do find it strange that although this is apparently a common problem, no-one seems to have an ansewer for it.
P.S. I have unchecked ALWAYS PUT THE COMPUTER TO SLEEP button in the Sleep Preferences panel. I understand that this is important. However, it has not changed anything.