I've worked with tons of these machines. Some are ultra noisy after all those years of service. Here's what you do.
First, you need to check the thumbscrews and be sure someone didn't screw them on ultra super tight. If super tight, they'll make the case vibrate! Not good!
The second thing you want to check is the fan on top of the heatsink. That could have some funky loud problems. That you can just take out, without risk to the system. It's very easy to take out. Just slide the plastic thing, that holds the fan, out of the little bar that holds it. You'll understand when you see it.
At this point you should be good to go. It might be a bit loud, but nothing extreme. The only thing you can do beyond here is replace the fan that blows air out of the computer. Any fan will do.
Unrelated, check the CD-rom to be sure if it works. PowerComputing put crappy CD-rom drives with their systems. They have a very high failrate after a few years.