Every year, in late summer, in Maine, at night, I hear a noise coming from the trees. It sounds like a cardinal's chirp (http://www.allaboutbirds.org/guide/Northern_Cardinal/sounds, fourth sound down), but longer, scratchier, and lower pitch. The sound starts and ends abruptly. There are three to six chirps in a row from the same tree, and then silence for a few minutes. Sometimes I will hear it again after a few minutes from the same tree, and sometimes from a different tree. No one else in my family can hear it. Does anyone know what this is?
This is my answer to your question, I am new here but I really enjoy this type of thing. In 2011, we had a very bad tornado come through our property. After that I have heard 4-5 new bird calls, which I have never before around here (Tennessee), and 2 at night. If you think back to when you first heard this bird, had there been any bad storms where you live? I hope this helps, I've been an outdoorsman all my life and know how frustrating it can be to know how things are usually, and all of a sudden something new comes along and no-one else has noticed.