OS X :: Humming Sound Coming From Early Intel IMac?

There is a new humming sound coming from my parents iMac, I think it's the processor cooler but I'm not sure. It could be the hard drive as well, I just know it kicks up sometimes when there is a lot of activity. The problem is they don't do a lot on the computer so it scares them.

They are convinced that it is going to die because it's soo old, I think under two years, but this is the second computer they owned. The first was the first 20" G5 iMac that, instead of turning it off the correct way, they just unplugged from the wall outlet. Needless to say it died, but they think it was because their son in law used it too much. I try to tell them that it's because how they turned it off, but they didn't like their son in law, who is now an ex-son in law, so they blame it on him. Yes, I know, my parents are the last people to own a computer.

They also don't want to spend any money on their computer, so worse case scenioro, taking it to the Genius Bar without this computer being under warranty and having them replace the processor fan and/or the hard drive, what would it cost? I think it might just require a new fan for the processor cooler, but what if they need to replace a lot more?