Good luck with your troubleshooting.
If you don't see anything obvious while you are looking around with the case open, try running the computer with the case cover off (or side-panel off) - that can help you hear exactly where the groaning noise is coming from. Probably the hard drive, but maybe not.
If you have a large magnifying glass and a strong flashlight, that can help with looking for leaking capacitors and such.
You can download the disk diagnostics for your Quantum from
http://www.maxtor.com (which bought Quantum). I think the Quantum diagnostics are under something like "Legacy Quantum drive software". I can look tomorrow for the link (it's late here in L.A. at the moment) . . .
Again, good luck!
. . . Gary
[hmm . . . one other thought: while the case is open, take a peek to see if you can see the name of the power supply's manufacturer. . . . and hope that it's not "Deer" ]