You found the poodles core, It has to be those files here: https://imgur.com/a/YRNG1Wn
I didn't try renaming those files yet, but I am not quite sure that they are the ones used by GSX. I also suspected them by name but they sound a little different... But I'll try later.
Anyway, in my case I can state that
a) GSX for some reason (either by design or by accident) uses a small plane piston enginge sound for the baggage trucks. When the baggage trucks leave, everything is ok.
b) this sound is
unaffected by the volume setting in the GSX menu
c) this sound file sits somewhere in the P3D \sound folder
I wasn't able to figure out how to set a different sound for the baggage trucks.
Anyway, I'll try to identify the exact sound file later and deactivate it.
But maybe one of the devs wanna chime in here....?