A proposal to spread the noise from aircraft leaving and arriving at National Airport by redirecting many flights over area communities now largely free of planes was disclosed yesterday by the Federal Aviation Administration.

The main announced purpose of the proposal, which the FAA said it is offering for public debate without recommendation, is to reduce the severe noise impact from frequent overflights that is now felt in several communities flanking the Potomac river.

This now results from planes taking off from National and following the Potomac River Valley for about 10 miles north of the airport and five miles south before turning and heading toward their destinations. Under the new plan, the planes would turn from the river in a more random pattern much sooner after leaving the airport.

"We are not trying to picture this as nothing but good news." James T. Murphy, director of FAA's metropolitan Washington airport service, told a Capitol Hill breifing session for members of Congress and their staffs. "There are tradeoffs . . . a lesser concentration of noise."

The plan will receive its first public presentation today at a board meeting of the metropolitan Washington Council of Governments (COG), which has scheduled five community forums starting march 26. COG is expected to recommend a decision to FAA on the proposal in June.

Basically, the plan would use a sophisticated noise evaluation technique and was produced for FAA under a contract by Mitre Corp. of McLean.

Under current flight procedures, the greatest concentration of aircraft noise is felt up and down the Potomac Valley insuh communities as Alexandria's Old Town and Georgetown, and at the points where planes turn away from the river - notably McLean and Cabin John, which are upstream, and Mount Vernon and Fort Washington, which are down-stream.

The proposed plan would reduce overflights in those communities, but overflights in those communities, but would add flights notably in these new areas:

The northern and western sections of Arlington and adjacent sections of Fairfax County and Falls Church; the extreme western and northedn tops of the District and adjcent areas of Montgomery County; the Franconia and Springfield areas of Fairfax County just south of the Capital Beltway, and the section of Prince George's County between the Beltway and the District line.