I have owned an awesome Apple Powermac G4 for about two years now. It's a Dual 1Ghz machine, which is commonly known as the Mirrored Drive Door (MDD) model due to its mirror finished optical drive doors. It has been a faithful servant so far for all sorts of stuff like video editing, music production, photos, web surfing, email and business duties, to name just a few.
I noticed when I first got it that it was a bit noisier than my old G4 533 Power Mac, but until my girlfriend's mother complained that it kept her awake all night, I didn't really take much notice of it. After that, though, it started to drive me crazy and I started a long campaign to quieten the thing down.
First thing I did was search Google for information on how to do it, and I found this page, which documents in detail how to replace the main 120mm cooling fan.
After trying one fan that just didn't cool the G4 well enough, I found (on FrozenCPU.com) a SilenX 120x120x38mm fan (18dBA; 90CFM) which is whisper quiet. It also cools the Mac just as well as the original Delta fan that came from Apple, only it's about 300% quieter. It made a big difference.
I also read about a software modification to Mac OSX called Nap Mode, which to cut a long story short, basically cycles the processors down into a "mini sleep" sort of mode, reducing the temperature and therefore the noise from the fans is quieter. This made a massive difference, but also added a new noisy high-pitched sound from the power supply fans. Nap mode can be installed from For G4 MDD owners, you must download version 3.5.2 of CHUD from this page; later versions do not work with MDD Powermacs. Some other models of Macs come with Nap Mode enabled as standard, like the G5 Power Macs and the PowerBook G4, so it's not a risk to use. Read more about nap mode on the MDD.
After more research, I found that another main source of noise is from the standard power supply fans that come with the MDD G4. So I chose to replace these with the fans that come on the Thermaltake Volcano 5 heatsink and fan combination -- the Everflow model F126025DH, which spin at 4550rpm with a maximum airflow of 32CFM and a noise level of 31dBA. These are actually the same fans as recommended on the above instructions at xlr8yourmac, so I was confident that they would do the job. The installation is not for the faint hearted, so if you're not good with electronics and fixing things, I wouldn't attempt it. This made a big difference once again.
I also chose to put a piece of material between the power supply and the case, as described here.
After all these measures, I now have a Powermac G4 MDD that is whisper quiet. A full description of this process is posted on my blog, if you need more details.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070223194451975