Had a similar issue a few years back in an apartment. To me it sounded like a helicopter landing (Woop,Woop). Got the maintenance crew coming and while I was hearing it they didn't. I must have been perceived as a bit crazy, but I persuaded them to come back with an db meter. Turns out the sound was there but on such a low frequency they couldn't hear it but thankfully the db meter picked it up. Culprit was a bad bearing in a fan on a rooftop on the adjacent building. That was what created the resonance I heard. But finding the sound pollution source isn't always easy. Wish you the best of luck