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It sounds like my neighbor's damn air conditioning... It's an Air B&B rental. They property owner has TWELVE industrial-sized compressors mounted to the side of their house and it vibrates like crazy. I can hear it throughout my entire house, but it's the worst in my office, of course!. So annoying!
Thankfully I don't hear it when they shut their A/C off. Unfortunately, I live in the tropics and whenever the unit is rented out, the visitors crank the A/C as high as it will go, in an attempt to escape the heat. When the place isn't rented, I have silence. Too bad I live in a very popular tourist destination, so the place is usually rented out. When it's rented out, it's makes me crazy. , It is a terrible sound. There's no way to block it out, it vibrates through your head and seems to resonate.

that really sucks challism

I hear it. Or I have in certain places.
Started when I lived in New Mexico many years back but I’ve heard it since. Mostly in mountains. Himalayas. Spain. Wales.
I actually like it. Like this deep sound that exists no where.
But I can imagine why people freak out.

Just to give you an idea I originally thought it was like heavy machinery in the distance. That kind of noise.