Eastern Screech Owls make a weird clacking or ratcheting sound as well..
I have had them in my back yard before and have heard the sound you describe.
I could only find this one video of it but maybe this is close?
http://www.youtube.com/watch?v=c4hiYqWEnZA#t=30
On this link scroll way down to the Eastern Screech owl sounds and hit the second play button..(Eastern Screech-Owl Screech Call and Chuckle Rattle)
http://www.squidoo.com/eastern-screech-owl#