[James]

I managed to free up JUST enough space to be able to defrag. The scratchy noise happened off and on during the process.

It continued to happen afterwards, so I turned the computer off, opened up the case, and blew away some of the dust inside. It's not sitting open and running.

I didn't hear anything yet, and was JUST going to say I think everything's OK now, but I just heard it again. It definitely sounds like it's coming from the power supply fan, so I'll call Dell and ask them to send someone out to replace it.