The brakes on Metro's subways make more noise than some jet airplanes. $1They are so loud that people stick their fingers in their ears when the trains screech into a station. They are so loud that two federal agencies, an assortment of brake manufacturers and one outside consultant are investigating.

There is no cure in sight.

"I don't think we're doing a very good job on the brakes," said Metro General Manager Richard S. Page, his tone reflecting frustration. "It is a serious and offensive problem. I ride it every day. I don't like it. I agree with every citizen who complains about it."

The problem first became noticeable late in October, when Metro began installing new pads on the disc brakes that stop the trains. At first it was just a train or two, but by mid-November almost all the trains were screeching, and they have continued that way.

How loud is it? Johnny Carson's audience might ask.

The sound can measure as high as l00 A-weighted decibels, equivalent for the few seconds it lasts to the noise of a jackhammer, and louder than a Boeing 727 jetliner taking off a quarter of a mile away. An A-weighted decibel is a unit of sound measurement that emphasizes noises heard by human ears. Each increase of 10 in a decibel reading is perceived as a doubling of noise, according to acoustical experts. Thus, 100 decibels would sound about twice as loud as 90 decibels.

"The Metro noise is a pure tone -- like running a piece of chalk on a blackboard -- and becasue it is a pure tone it is perceived as being even louder than it actually measures," said Charles Elkins, who grinds out noise regulations for the Environmental Protection Agency.

There are no federal standards for safe levels of subway braking noise. The only regulation that comes close is an Occupational Safety and Health Administration (OSHA) standard that says that noise in the work place must not exceed an eight-hour average of 90 decibels, a standard that clearly permits occasional readings about 90 decibels.

Eight-hour averages have not been established for the Metro system, so it is not known if Metro's station attendants and train operators are being subjected to excessive noise levels. ISHA, however, has received complaints from Metro employes, an official confirmed.

The 100-decibel reading was recorded on the lower platform of the L'Enfant Plaza Metro station the other morning by a respected federal noise expert.He agreed to accompany a reporter and take noise measurements on Metro provided his name does not appear in the newspaper.

That was the loudsest of many readings taken at various points in the subway system, but there also were brake noise readings of 99.9, 98.6 and many of 95. Metro itself has taken more than 1200 readings that recorded brake squeal between 75 and 95 decibels.

Furthermore, the federal expert said, "This particular sound is at a frequency that is most annoying to humans. It is stressful and irritating. But because it lasts for only trwo or three seconds at a time, it is probably not harmful."

The noise in the subway station itself, with no trains present or braking, averaged about 63 decibels. Noise inside the trains as they were running was measured between 75 and 90 decibels, depending on such factors as speed, whether the cars were going around a curve and whether they were running inside or outside a tunnel. Running in a tunnel added about 10 decibels. The musical "ding-dong" signal that precedes door-closing on the trains measured consistently at 92 decibels.

Braking noises measured about 10 decibels less at outdoor platforms than at underground ones.

The typical living room, with no stereo or television going, measures about 45 decibels. A typical office measures 60 decibels, and jumps to 65 when typewriters are added. A busy street corner is 75 to 80 decibels. A bus accelerating from a curb records 85 to 90 decibels. A Boeing 727 jetliner taking off, when measured from the side at a distanced of a quarter of a mile, records 91 decibels.

Metro's maintenance engineers, federal consultants and brake pad manufacturers are attempting to figure out how to fix the problem and why it suddenly came up, after four years of relatively quiet train operations. Like all noise, the Metro shriek is caused by vibration, in this case the vibration produced when the brake pads are pressed against the brake discs.

Metro cars brake in two phases. The first phase is called "dynamic" braking, and involves electrically slowing the motors on each axle. Dynamic breaking is effective to about 15 m.p.h. At that slow speed, disc brakes take over.

A steel disc, 20 inches in diameter and 3.6 inches wide, is affixed to each wheel and runs between two brake pads. The brake pads are compressed under hydraulic pressure and the friction of that action stops the disc, and thus the axle, from turning. In addition to the pads and the discs, the brake system contains plates, pistons and two layers of sound-deadening materials.

The pads cost about $25 each and must be replaced about every 14,000 miles or 14 weeks. The discs cost about $325 each and last for about 25,000 miles, or 25 weeks. Pads and discs are inspected daily and replaced when wear exceeds specified limits.

For the first four years of Metro operation, the pads came from the Abex Corp., which built the Metro braking syste. The Abex pads contained asbestos and the firm decided to discontinue manufacturing pads containing asbestos, although it does make a substitute pad, according to an Abex official. Asbestos, a very effective braking agent, has been found to be cancer-causing in some forms and is particularly difficult to handle during manufacturing.

When Metro took bids for new pads, the Knorr Brake Corp. won the contract. Knorr pads, also without asbestos, have been installed on Metro cars since October. Metro's Erich Vogel and Knorr's Stefan Rusynko said the noise problem with the Knorr pads did not appear during Metro's safety and durability tests.

Vogel has been experimenting with different pads from several manufacturers, including Abex and Knorr, to find one that will stop the tgrain safely, last a reasonable amount of time and be quiet. It has been decided that pads containing asbestos will not be used and, in fact, they are not generally available. Federal research agencies and an outside consultant are helping Metro.

All express confidence that a short-term solution -- a quiet pad will be found within a few weeks. Once new pads are available the entire Metro fleet could be fitted in about two weeks.

Beyond that, Metro and its consultants are studying the braking system to see if a different technical approach to the disc brakes might not be the long-term answer. No other subway system has a braking arrangement exactly lke Metro's.

Metro officials say they cannot estimate the total costs until they know prescisely what they have to do. A one-time change of all brake pads, however, would cost at least $135,000, so the solution could easily reach several hundred-thousand dollars.