San Antonio/Bexar County has only been historically on the edge of the range of the Giant Cicada "Chicharra". For the past few years they've been expanding their range into the Hill Country and now this is what you hear outside along with the usual dog day cicada. They are pretty awesome sounding but usually impossible to see in the tops of the trees. If you go farther south towards the coast and Brush Country, you can stop your car on the side of the road and the noise of these things is so loud outside you can hardly carry on a conversation. They are about 2 to 3 inches long.
http://www.texasento.net/Cicada.htm#Sound
http://www.texasento.net/Cicada.htm#Sound