The brakes on Metro's subway cars, unhappy about something Metro's mechanics do not understand, have taken to shrieking.

"It sounds just like subways in New York and Chicago," complained a Carter administration appointee who did not want to be identified while he is looking for a job. "I thought I'd gotten away from the stuff." He used to live in New York.

The problem has become increasingly noticeable to Metro regulars over the last two or three weeks. At first, an occasional train would emit an ear-splitting noise while braking. Yesterday, the trains were doing it in chorus. Three riders called The Washington Post to complain.

Ralph Wood, who is in charge of running the subway for Metro, agreed that the noise is awful and said that the problem is baffling.

"We don't understand why it's worse now," Wood said. "We're running some tests to find out." Four cars with specially prepared braking equipment were scheduled to run around Metro's tracks last night while technicians recorded braking noise.

There is no safety issue in the squealing brakes, Wood said. "They're safe," he said, "just noisy."

The source of the shrieking noise is the disc-brake system. When Metro's trains brake to a halt, they do so with the help of two large brake pads -- each about 18 inches in diameter -- being pushed under hydraulic pressure against a disc on each wheel of the train.

Metro (and brake pad manufacturers) have been looking for a long-wearing, quiet brake pad material since they decided to stop using asbestos because of the cancer-causing potential of asbestos dust. Asbestos was used on Metro's cars when they first started running in 1976, but only a few of the asbestos pads remain in service. Environmental tests in Metro's tunnels have found no health problems with asbestos dust levels.

Substitute brake pad materials, however, are creating noise problems. Contributing to that, Wood said, are the facts that Metro has changed suppliers of brake pads recently and is now using two different brands. Trains that have brakes with both bands of pads apparently make more noise than those that just use one brand, Wood said. Additionally, he said, mechanics apparently left out sound-deadening materials during the installtion of some new pads.

The pads cost $25 each and are normally replaced about every 24,000 car miles, just short of five months. The brake discs cost more than $300 each and are replaced about every 28,000 miles, just short of six months. Pads and discs are inspected for wear every two months.