Federal Aviaton Administration officials confirmed today the agency is looking into allegations of deficiencies in Northwest's maintenance of DC-9 aircraft. Airline officials say the agency hasn't told them of any current investigation. The airline says allegations have been made, investigated and settled, and the F-A-A has raised no concerns about safety. Minnesota Public Radio's Bill Catlin reports. The investigation is the focus of a front page story in the Wall Street Journal, which notes that the airline has saved billions of dollars by upgrading elderly DC-9's instead of replacing them with newer aircraft. The article quotes an F.A.A. official saying the agency has received allegations that appear to have some credibility and th