This video has been uploaded from Moscow, Russia, on March 19, 2012. The sound had a low volume so earphones are actually needed.
The author says:
I’ve heard this sound at 15:30 MSK. I thought it’s a propeller airplane first. Then the sound started to swell and change its pitch. I’ve been seeking for a camera battery about 5 minutes. When I was ready to record the sound started to fade out but I still had time to capture it.
