Powerbook back...
It arrived back again last Friday. In the box was a single piece of A4 which includes something to the effect "it wasn't broken, so we didn't fix it".
At least part of that is right - they didn't change the hard drive. Within 30 minutes of turning the machine back on it was making the stupid noises and hanging again.
Back on the phone to Apple support. They don't think that there's anything wrong with the machine, and have compared the noise it makes to other Powerbook G4's they have in the workshop.
All this really indicates is that they haven't heard the problem. Running through Disk First Aid's verify and repair cycle a few times, I can get the machine to the point where it claims that the disk doesn't actually exist.
They're picking it up again on Tuesday (tomorrow). This time I'll include instructions on how to reproduce the problem, including a note to the effect: "If you can't reproduce, don't just send it back - ring me on ...".
We'll see. Almost a month now since I reported the problem.