Something I've been doing lately  Nearly every time we go out in the yard when we get close to the woods we hear one tree knock.  If we walk out the old road behind the house a ways out we hear another single knock. So I've started when I get close to the woods I will clap my hands loudly.  Some times just once, sometimes three times. When I do this I never hear the single knock. Ever. I have no idea what this means, I just find it interesting. 

If the single knock means humans coming (which is what I think it means)  then why not do it when I clap? 