Trying to Identify This Bird by Its Call

I have tried to find what bird is making this sound in my yard. I have yet to be successful. If you have an idea, please comment. Until then, this post will be labeled....Unidentified Bird. :)

The Handbook of Nature Study, page 42:
"Anyone who attempts to recognize birds by sight alone misses much of the pleasure that comes to those who have taken the time and pains to learn bird songs and use them as a means of bird recognition. It is true that not all people have a talent for music; but anyone interested in birds can learn to identify the songs and most of the call notes of common birds."

I am learning this to be so true in my life. I love being able to hear a call or song and know who is making it. This is a lifelong project.

Barb-Harmony Art Mom