Just in case anyone was wondering - Gerotor, as used to describe an electric pump in my earlier post, was not a typo. Gerotor pumps are not the same as gear pumps. They do have their own distinctive normal sounds and, as I found out, a quite different sound when they are failing.
https://en.wikipedia.org/wiki/Gerotor
https://en.wikipedia.org/wiki/Gear_pump