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Weird behavior is possible if the machine overheats, but I'd worry first about the hard drive dying -- because (A) you're having the problem at startup and (B) if the drive dies, you're about to lose everything on it -- and it could cost hundreds of dollars to recover everything, if you haven't been backing up onto an eternal disk using (say) Time Machine
Here's what I would do in your position -- and I would do it, like, immediately because things probably are only going to get worse
(1) If you haven't been backing up regularly, get a USB flash drive (say) and copy your absolutely most important files over immediately
(2) If you haven't been backing up regularly, get an external hard-drive and copy the other files you want to it, either manually or by running TM
(3) Buy a new hard drive and search the apple site for the hard-drive replacement instructions for your model. Here's an example for the late 2004 model: manuals.info.apple.com/en/PMG5_HD_DIY.pdf
(4) Replace the harddrive and reinstall OS X. If you got a successful Time Machine backup, you should be able to restore everything onto the new drive. Otherwise, you're be re-installing a lot of programs by hand
If you want, you can use Disk Utility to check the SMART status of the drive after booting from the OS X install disk: if it tells you anything other than that it has successfully verified SMART status, you'll be pretty sure the disk is in trouble; but there can be disk problems that the SMART status verification misses
WARNING: I am a complete amateur at this. I pay attention to my own machine, and I'm telling you how I would react if I had your problem, as you've described it. When you talk to your tech person, try to be sure to tell that person everything you know about the problem (like the loud noise!) -- it really will help
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