They start singing about an hour after dark, calling with short bursts of high-pitched sound. Each call is somewhat like a can of dried peas being shaken rapidly. In the following recording, the lesser-angle wings call five times. You’ll also hear the common true katydid singing “di-di-did” in the background. Crickets are also singing in this clip — they form a continuous band of sound against which the katydids play.
For comprehensive identification information on America’s many katydids (including sound files) see Walker and Moore’s excellent online guide.