Strange Noise and Bright Flash in the Sky in Minnesota – Military test?

This strange noise and the mysterious bright flash happened on September 25th, 2011 at a small lake West of Alexandria, MN.

The constant hum/rumble lasted for around 4 minutes. The sound changed slightly just before the mysterious flash and then faded away. But what was this unknown light? Military trainings? Ball lightning?

The strange phenomenon occurred at around 2pm. The sky was very cloudy but no rain or thunder/lightning.

Another witness believes the sound was a transformer blowing up but he didn’t see the flash.

Watch the video – The flash is at 1:16 but the rumbling occurs randomly throughout..

The uploader thinks it is something military related.

Any ideas? Ball lightning?