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?