I witnessed something similar some years ago, in Lisbon.
Our living room had a window facing the street, and sometimes one could hear a low pitch humming noise that made that window shake. The first time it happened I remember wondering who would leave a delivery truck in neutral for so long.
It was really annoying, especially because when I opened the window, there wasn't a car parked outside - everything was silent. Until you listened carefully, only to confirm the hum was still there, seemingly at a distance.
So the window was reverberating to the hum. But where did it come from? Across the street there is a military academy, so I guessed the obvious guess: military experiments, of course!
The problem with that guess was that under closer inspection, the noise didn't really seem to come from that direction. Usually, one could only hear it in the afternoon, but the hum was present all week round - and around there only the military worked weekends. But it just looked it came from in between some trees far away. So I guessed differently: a big air conditioning system, some large boat, a generator in some construction yard, a concrete mixer...
The hum's amplitude would slightly vary, and the window would just hum along or even rattle annoyingly sometimes. So I guessed that if wind was a factor in modulating the hum's intensity, it would be relatively far away. Either that or it was coming from a moving source.
One day the hum simply stopped. I never found out was was behind it.
Our living room had a window facing the street, and sometimes one could hear a low pitch humming noise that made that window shake. The first time it happened I remember wondering who would leave a delivery truck in neutral for so long.
It was really annoying, especially because when I opened the window, there wasn't a car parked outside - everything was silent. Until you listened carefully, only to confirm the hum was still there, seemingly at a distance.
So the window was reverberating to the hum. But where did it come from? Across the street there is a military academy, so I guessed the obvious guess: military experiments, of course!
The problem with that guess was that under closer inspection, the noise didn't really seem to come from that direction. Usually, one could only hear it in the afternoon, but the hum was present all week round - and around there only the military worked weekends. But it just looked it came from in between some trees far away. So I guessed differently: a big air conditioning system, some large boat, a generator in some construction yard, a concrete mixer...
The hum's amplitude would slightly vary, and the window would just hum along or even rattle annoyingly sometimes. So I guessed that if wind was a factor in modulating the hum's intensity, it would be relatively far away. Either that or it was coming from a moving source.
One day the hum simply stopped. I never found out was was behind it.