I am really curious now! They are (obviously) both metamorphosed, correct? Has there been a large temperature change lately? Perhaps some odd, unknown mating ritual? Haha.
Just a side thought, you are sure that they are both females, correct? I know males sometimes have a tendency to get more aggressive. They are not fighting? Just chilling on a log?
Do you find that they begin making these noises when you watch a certain channel on the telly? Perhaps when their favorite team starts losing? I'm teasing. But I honestly can't wait to hear that! Or even a video courtesy of youtube?
Cheers-