Clicking Sound coming from G5 Tower

[jsundgren]

I just purchased a used G5 computer from Craig's List - went over to the guy's house, took a look at the computer and everything looked/sounded great. Somewhere in the transfer, a piece got loose inside the computer and fried the video card when we tried to start it up. Got that fixed, but now the tower makes a loud, consistent clicking noise. I have no idea where it's coming from, but it doesn't sound good (not to mention it's extremely annoying). The computer seems to be working fine except it does not recognize any of my DVDs (I use DVDs to back up a lot of my work). It opens CDs fine, but not the DVDs. I've contacted the guy I got it from and he has no idea what it could be - he says it worked fine for him before I got it. Please help! I don't want to keep using the computer if there's a serious problem with it. Thanks in advance.