TESTIFYING before a congressional committee the other day on the future of National Airport, Marilyn Cable described a 30-mile stretch of the Potomac River as a "sewer of noise." She's right, of course, and almost everybody knows it, including most or all of those 67 representatives who have signed a resolution demanding no reduction in air service at National. But in case some of those 67 don't know it, we invite them to join us for a couple of "quiet" hours along the river late some afternoon.

Among the places we might choose to take them would be the riverfront in Alexandria, Lady Bird Johnson Park (sounds quiet, doesn't it?), the Lincoln Memorial, Theodore Roosevelt Island and the C & O Canal towpath. We might even add to our tour the back yards and balconies of some of the thousands of dwellings along the river. If we did, we would also insist that our guests try to comprehend what is said on the 6 o'clock news without turning the TV volume up to screech-level as the planes pass overhead.

The mere extension of this invitation is, of course, its own testament to our belief in the good faith of the awful 67. Surely members of the House (and some of their colleagues) would be more sympathetic to the case made for reduced air service at National if only they understood the extent of the intrusions that this "sewer of noise" brings daily into the lives of tens of thousands of people.

Or . . . would they be? There is some evidence -- other more cynical observers would describe it as overwhelming -- that many members of Congress don't give a damn what price is paid by residents of Virginia, Maryland and the District of Columbia so long as they themselves have the privilege of doing their politicking with the assistance of jet airplanes that land conveniently (for them) near the Capitol.

At least one observer -- business consultant Harry J. Donohue -- seems to think that is the way a majority of Congress feels. Writing in The Washington Star last Sunday, he suggested a wonderful solution for National Airport: not only should it be renamed -- it should also be closed to all planes except those bearing members of Congress. Give Sen. Throttlebottom and Congressman Harumph their own publicly supported private airport. They have so many perks already that another wouldn't hurt, and it would get the noise along the river down to a bearable level.

We love the Donohue solution, but we're not quite ready to endorse it. There is still a faint hope that members of Congress will be decent, fair and honorable about this matter. Why, even the chairman of the House subcommittee that held the hearings thought members should withhold judgement until they heard all the evidence.

Unfortunately, the evidence we think they should hear cannot be presented in a quiet room in a House office building. It is the evidence of the noise itself -- at Gadsby's Tavern and Carlyle House, in Foggy Bottom and Rosslyn. That's why we are inviting the 67 representatives who signed that resolution to come with us and listen.