I'd guess that you're right - probably a heating pump, dehumidifier, or some other motorised gadget.
With things that have juddery moving parts, vibrations can get into the walls or floorboards, making your side act a bit like a loudspeaker - so you're maybe not hearing the sound directly, but from tiny vibrations of your ceiling.
If it's something which is stood on the floor (and your neighbour is amenable), placing it on top of a thick rubber mat should help to damp the vibrations - I used to do that with a dehumidifier in my old attic, and it cut down the sound loads.
_________________
When you are fighting an invisible monster, first throw a bucket of paint over it.