The T-37 is called the Tweet because of the extremely loud, high-pitched whine made by the engines. This whine is actually the sound of air being drawn into the engines.
The T-37 made its maiden flight in 1955, and went into service with the United States Air Force a year later. The plane was upgraded subsequently and continues to train pilots. It can also be refitted in order to perform recon missions and has been converted to an attack aircraft by several national Air Forces, particularly in Latin America.
The U.S. Air Force is presently retiring its fleet of T-37s in favor of the new T-6A Texan II, a turboprop with more power than the old Tweets and modern avionics.