Yet another mystery sound

["PaulD" paulcdickinson]

While recording in the Big Island Lake Wilderness area of the Upper Peninsula 
of Michigan, this very unusual sound popped up. At first, I thought it was RF 
interference, but the nearest broadcast facilities were 18 miles away in 
Munising, plus the sound has an airy quality, and comb filters as it travels 
from left to right. It sounds airborne.

http://tinyurl.com/2fcuhlb

Time: approx 11:00 PM, 6/20/10. Location: thicket on the shore of a small 
glacial lake surrounded by middle-aged maple/birch/poplar/evergreen forest.

Any thoughts as to its source would be most appreciated.

Thanks!

Paul Dickinson