Clacking of slow-moving, elevated light rail trains, from a distance
Cicadas, in the summertime (audio)
Frogs, at night
Male woodland cicadas (Tibicen platypedia?), in redwoods: "...from male woodland cicadas, sending out their mating calls with "a cacophony of mechanical-sounding clicks." These little insects climb high into the trees, so that they're often heard but rarely seen." (via)