When you shake the disk drive, a rattling noise is normal. It even says so on the disk label.
However, if you hear a rhythmic ticking noise during regular operation of the disk drive (when you're not shaking it), you could be looking at a drive that's either already bad or ready to go bad.
When my 4gb disk drive started making that noise several months ago, it failed completely within five minutes after I heard the noise the first time. It's time to write down your EQ settings, make sure all your MP3s are ready to re-install, and start looking at pricewatch.com.
It is also possible that you've got a loose cable or a loose slave jumper, so check those.
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Tony Fabris