Neighbours have a weird sounding machine i cant figure out what it is

The neighbours up above have some kind of a weird machinery which randomly makes a loud, buzzing sound that is hard to describe. It's not vibrating but more of a humming. It lasts for a few seconds only at a time so I guess you could compare it to an elevator of some sort or some kind of an elevation-machinery is the only thing which comes to mind for me but I don't know. Some nights though it can be heard many times and those same nights I can hear the neighbours being up and about, walking around, going to the bathroom and talking. You can hear those things when it's the middle of the night and completely quiet all around. But usually it's only heard once for a few seconds like a low frequency buzzing at random times throughout the day. I've never heard it before with any othr neighbours but those are odd neighbours. They are old and for a long time they used to have the TV on at the loudest possible volume ONLY at nighttime so it was impossible for me to sleep and I had to leave a complaint TWICE before that stopped. This sound isn't really disturbing though.