Eerie Sounds in the Night

If there is an unexpected sound at night, 

I can't go to sleep until it's identified. 

It happened last week....

I crawled into bed.

All the house lights off.

The blankets tucked under my chin. And all was quiet.

...until

A humming sound came from outside. It didn't really sound like a machine, more of a choral men's group singing one note, no two. The melody broke into harmony changing pitch.

The source of the sound mystified me. It was a blistery cold winter night. The sky gray all day turned charcoal at night. 

The snow was wet in some places, crunching beneath out boots that day, and dry in others whisking up in the air with the wind.

The tones changed to a ghost like call. The sound the wind can make as he sweeps around the corner of the house. 

The humming increased in volume, still pleasing to hear. The constant nature eliminated the wind. It had to be a motor of some kind, but which one?

After I show you, you may not agree that this could make the sound described above. But perhaps it was the night in question. The quality of the air, the humidity, the very rural location, and the hour.

What made the sound? The next morning I captured it, took the photo out my living room window:

The next time you have an opportunity to listen to a snowmobile from inside a building, see if you can hear a men's choral sound. 

What mysterious sounds have you heard in the night? 

Have you identified them?