High-pitched buzzing/humming noise from above
I've been hearing some high-pitched buzzing/humming coming from my upstairs neighbors' unit.
When it ends, it is followed by a loud bang. Before it starts I hear some fan or rotor blades shrieking.
The buzzing/humming sounds lasts for several minutes. It seems to come on at some regular intervals. It starts only around 9 p.m. every night.
I had approached my neighbors before about this noise but they pretend to not know about it.
Does anyone have any idea what could be causing it?