[pct-l] Scariest Sound

[ECPG at peoplepc.com (Carolyn Eddy)]

I heard a similar noise once which totally freaked me out. My brother, the 
mighty hunter, says that grouse make that noise when they are dumping snow 
or dust on their wings to get off mites, etc..

Not sure I buy it but your description is exactly the same as the sound I 
heard.

The area you were camping in is prone to drum circles and weirdness. Could 
have been that too.