This strange humming sound has been recordered in Warsaw, Poland, on May 24, 2012. The author says that it wasn’t an alarm or a plane.
The author comments:
I have recently read about all those strange sounds around the world and suddenly I heard something similar at night. It sounds kinda different from most of sounds in various videos but still they where untypical. That wasn’t any alarm or plane. They started after 1 A.M and lasted for over 40 minutes. Then they stopped for 5 minutes and started to howl again but not so loud this time.
I recorded them with my mobile so don’t expect a good quality.