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.
The APU is in the TAIL of every commercial airliner. You could never hear it as a passenger.
The PTU is under the passenger floor. It is found on most large aircraft. It is a hydraulic transfer pump that makes a noise that can be described as “grinding.” Although rarely operating, it is a perfectly normal system, and hearing it indicates the system is working to maintain hydraulic pressure at high transfer times, such at landing gear retraction. Hearing it is not a sign of poor design or poor maintenance.
Thanks for playing the Holiday Inn Express Arm Chair Expert.