As the federal government issued warnings yesterday of possible safety problems with 399 buses being used in 11 cities, including Washington, a Metro spokesman said that engineers in Washington already had found the defect and had corrected it themselves.

The National Highway Traffic Safety Administration said investigators have found that passengers standing in the stepwell of the buses are subject to possible injury when the doors of the buses are opened.

The agency has thus far learned of 56 accidents related to the defect, in which 10 persons were injured.

The agency said that AM General, the distributor of the West German-made buses, "had knowledge of the safety defect but failed to comply with statutory notification and remedy requirements" under the law.

However, Metro spokesman Cody Pfanstiehl said last night that while Metro had received no notification of the defect, "we had found several months ago that people's feet were getting caught in the area where the door was closing, so we put in a railing to correct the problem."

The defect has been found in articulating buses (double-length buses with a flexible connector in the middle) being used in Chicago, Seattle, St. Paul, Pittsburgh, Phoenix, Atlanta, Los Angeles, Oakland, San Diego, San Rafael, Calif., and Washington, D.C.

The buses are manufactured by M.A.N., a West German company.

Representatives of AM General, a subsidiary of American Motors Corp., could not be reached for comment.

Defects also were found by the agency in 85,659 Toyota Hi-Lux light trucks. The trucks, built in 1979, have been found to have a shimmy in the front suspension, causing drivers to "experience considerable difficulty in controlling the vehicle," the agency said.

Agency spokesman Ed Pinto said Toyota developed a steering damper to compensate for the problem. The 1980 and 1981 models have the damper as original equipment.

Pinto said the safety administration is aware of five accidents that allegedly occurred as a result of the vibration problem and reportedly caused four injuries.