45 years in aerospace and you think a grinding noise is an APU?
I think my question your observation is right on target. Google APU.
Like I said....didn’t do aircraft.
but...since you asked:
>https://en.wikipedia.org/wiki/Auxiliary_power_unit<
An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft, ....
The GEARBOX transfers power from the main shaft of the engine to an oil-cooled generator for electrical power.