Insect making continuous sound in summers
These are Cicadas, of the superfamily Cicadoidea of the order Hemiptera.

These sounds are made by the males to attract mates. There are a lot of species, each making there own specific sound to attract the right mate. However, to the human ear, these sounds don't sound different. Cicadas are also able to produce other sounds, in distress or during courtship and mating, but the continuous sound during night and day is to attract mates.