I spent the night of 27th of August 2005 near Seneca, Illinois in my van and kept the microphone on the roof. I heard about fifty whistles and then rock knocking. After the barred owl hooting the rock knocking started up again.

Click here to listen to sound clip:  Rock Clacking 1

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Spectral View

Click here to listen to sound clip:  Rock Clacking 2

Waveform View

Spectral View