Road noise usually. I live between two fairly busy streets and I like to open the windows for the breeze. Alternately, on 90+F days I close the windows and the air conditioner is loud.
And I also we have lots of lovely bird noise. From about 3am to 7am they are the loudest thing in spring and summer. Lots of sparrows, cardinals, and robins. Later in the day the big crows caw and the swallows (or swifts?) fly high and twitter while they rid us of bugs.