Now I know

After much research and asking many people we found out what the sound that is so beautiful to our ears comes from. We tried crickets, tree frogs, cicadas. We listened to the sounds of many on the web. Heard some that were close but as we repeatedly listened we realized some of the sounds were not quite it.

And then we heard it. We knew it as soon as we did for there is no other sound like it that we have ever heard.

Listen for yourself. http://en.wikipedia.org/wiki/Spring_Peeper#Vocalization

And if you can, try to listen only once. We could not

And we still have not seen them.