That is the sound a single acorn makes as it falls from the trees which overshadow our humble home. Bang! It hits the roof. Rumble, rumble, rumble. It rolls down the roof, and then Ping! It hits the gutter on its way to the ground. This unremarkable chain of events is enough to startle a person if the house is quiet, and wake a person if they are sleeping.
Anyone want to guess the effect that this many acorns has?