Who speaks for the people of the Washington area on questions of noise and traffic at National Airport? Anyone who knows kindly contact the Federal Aviation Administration.

The Metropolitan Council of Governments, composed of representatives from 18 local and state governments, last week voted in favor of a controversial test to reroute jets taking off from National Airport, spreading their noise for 320,000 more area residents to hear but giving relief to others.

Yesterday, however, the Transportation Planning Board, a COG affiliate on which each of those same 18 governments has a voice, voted to oppose the so-called scatter plan, saying that serious questions of safety and noise remained unanswered. Officials at the Federal Aviation Administration, which runs National and will make the final decision, are a bit perplexed.

"We really hadn't envisioned a situation where two different parts of the Council of Governments would end up saying two different things," said airport director James Wilding. Whose advice will be heeded? "I don't know." Wilding said.

Either way, Wilding said he expects to have a decision by mid-August. The proposed 90-day test would allow pilots to veer off the Potomac River shortly after takeoff instead of proceeding to Cabin John or past the Woodrow Wilson Bridge before leaving the river.

The planning board was created in 1965 to meet federal rules that areas receiving transit and highway aid engage in regional transportation planning. The board's approval generally is needed to obtain federal transportation funds. The board shares staff and letterhead with COG but beyond that the two agencies often have separate missions.

"You can't say that either is supreme," says a staffer who works for both bodies. "The best description is that COG is the umbrella organization and that the board works within it but at the same time has certain independent policy responsibilities."

Planning board Chairman Marie B. Travesky of Fairfax County yesterday rejected suggestions that her panel close ranks with COG and vote for the scatter plan in the interest of regional unity.

"That would be abdicating our duty," she said. "We have a mandate to look at the issues based on their impact . . . as we see them."

It was COG that set the ball in motion in 1981, asking the FAA to test a scatter plan.

In recent months, however, opposition has mounted among member governments, which argue the test would expose quiet neighborhoods to jet din and compromise safety by routing more planes over populated areas. The FAA has responded that the test could be conducted safely.

Arlington and Fairfax counties, the District and Alexandria have gone on record in recent weeks against it. Montgomery and Prince George's counties have reaffirmed their support.

Last week, in a 15 to 8 vote, COG upheld its support for proceeding with the test. D.C. City Council member H.R. Crawford (D-Ward 7) voted for it, despite the City Council's position against it, as did two representatives of Mayor Marion Barry.

Yesterday, the planning board voted 11 to 5 against the scatter plan. When members called for a "weighted" vote, in which population determines each jurisdiction's say, it worked out to 6.07 votes against the plan and 5.4 for it.

D.C. council member Frank Smith Jr. (D-Ward 1), who voted against the test, said that it would disturb Rock Creek and other quiet neighborhoods in Northwest Washington. "Those of us who live along there, including myself, are going to get an awful lot of noise," he said.

In case its advice doesn't stick, the board passed a resolution calling for its technical staff to take part in any test's evaluation.