Last weekend, we heard it for the first time as we were waking up on Saturday morning. At first, I thought it was the hum of faraway yard equipment. Then it sounded like some kind of quacking, but again, far away. Today, we got serious about trying to find the source of the noise and eventually we did--it's a muffled buzzing sound and it's inside the exterior wall of our bedroom. The exterminators come on Wednesday.