I tried different setups (and even recorded it) and it's strange.
I muted all GSX sounds in the settings.
Fresh situation, FSL A320X, cold& dark. When I call for fuel, the piston sound starts immediately (external view).
Reset GSX position.
Call for deboarding, everything is quet.
Reset GSX position.
Call for fuel, again the piston sound, but much lower in volume.
....?
I'm out of Ideas here.