Only vaguely related to the original article but I've often wondered what causes the loud screeching noise as an Airbus 320 shuts down. I did a bit of reading around it and came to the conclusion it is actually the PTU (basically a hydraulic motor directly coupled to a hydraulic pump which allows coupling the hydraulic power between the two systems without risking bringing down both in the event of a failure) trying to keep the pressure in one system up as the engine driven pump stops.

Anyone happen to know if I'm correct?