Yesterday I rided the DC-9 (Northwest Air) from MSP to STL. My seat is near the wing, and it was very noisy. So I did some search, and here is the wiki entry of DC-9. As I guessed, DC-9 is an old aircraft (30+ years old). Quote Wiki:
“The McDonnell Douglas DC-9 (initially known as the Douglas DC-9) is a twin-engine, single-aisle jet airliner. It was first manufactured in 1965 with its first flight later that year. The DC-9 was designed for frequent, short flights. The final DC-9 was delivered in October 1982.
……
Northwest Airlines operates a fleet of DC-9 aircraft, most of which are over 30 years old. With severe increases in fuel prices in the summer of 2008, Northwest Airlines finally began retiring its DC-9s, switching to Airbus A319s that are 27% more fuel efficient…”
The flight from Minneapolis/St. Paul to St. Louis was only half full. I guess NWA could use a smaller plane to save some fuel, and noise.