The most efficient way airports can operate is to land and depart aircraft on certain runways for an extended period of time. This means that while the winds are blowing from a constant direction the aircraft will always be flying into the wind. While approaching the airport, the aircraft will be in a line several miles apart flying into the wind getting ready to land. The repetition of aircraft you hear from the ground is the noise the "lined up" aircraft are making while on their approach or, in the opposite case, taking off.