G5, as noted alongside. Now takes an age to read a MacFormat cover disk into Disktracker, and on the occasions that it does show as finishing the job it shows disk contents of around 125MB, instead of more than 4GB!
Sounds: Never easy to describe
It starts up with a sort of growl, then as it reads the disk it seems to be stopping and starting with a sort of kerchunk, kerchunk, kerchunk. The Disktracker (early version running in Classic) progress bar shows the lack of progress - it will move a little bit, stick for a while and then move a bit more, though the kerchunk noise seems to be constant.
I'd just appreciate confirmation that this is not some sort of software or firmware problem before I get on to John Lewis's warranty department. Fortunately, when I bought it in January 2006 it came with a two-year warranty.