The administration's proposed policy to control noise and congestion at National Airport in effect won strong support on the Senate floor yesterday, raising hopes among advocates that most elements of the controversial plan will be implemented soon, perhaps by the end of this month.

By a vote of 77 to 15, the Senate passed a $10.4 billion transportation appropriations bill, upholding a committee decision to delete a House amendment that would bar any reductions in flights at National below the level of July 31 this year.

In the House, meanwhile, the two congressmen who sponsored that amendment said they had dropped their support for it, because airlines that had condemned the policy as anticompetitive have now reached an accommodation with the U.S. Department of Transportation, for changes in the plan. The amendment was aimed at the plan's limits on air traffic.

Sen. John Warner (R-Va.) expressed optimism yesterday that the amendment would not appear in the final conference bill that both houses of Congress will have to approve. Conference and final congressional action could come this month, with the Transportation Department planning to implement the policy on Nov. 30.

Transportation Secretary Drew Lewis and Washington area legislators who have campaigned hard for the policy in recent weeks hailed yesterday's Senate vote and expressed hope that the tide has been turned in the policy's favor."I think we're on our way," said Rep. Frank Wolf (R-Va.), who coordinated support in the House.

For close to a decade, civic groups in neighborhoods below National's approach paths have campaigned for a comprehensive plan to control growth at the busy airport. However, Congress last year blocked such a plan -- many legislators fly out of federally operated National frequently and are wary of plans to reduce service there.

As outlined in July by Lewis, the new policy would reduce landing and takeoff "slots" for commercial airliners from 40 to 37 per hour; cap passenger levels at 16 million per year; impose decibel limits on engine noise, and upgrade roads and transportation to Dulles International.

Lewis said it would also close loopholes by which some airlines, primarily New York Air, had been operating well above their formal slot allocation. This feature caused New York Air to charge that the plan would be anticompetitive and would effectively shut it out of the market.

New York Air and other airlines lobbied energetically against the policy before the House passed its amendment in September. But in subsequent talks with the Transportation Department, New York Air reached an agreement for changes in the policy that led it to call off its campaign.

Yesterday, an administration source said that the compromise provided for flight reductions at National to be distributed proportionately among airlines, based on the actual number of flights each airline was operating in late July, not on the official slot allocations.

This would be done, however, only if the airlines could not agree among themselves on how many flights each should be allowed.

Phase-in schedules for the policy's limits on decibel levels for aircraft engines will also be altered, the source said. As originally outlined, the policy would immediately establish daytime and nighttime noise standards, to be placed at levels that airlines could meet with current equipment.

However, because airlines have raised technical objections to the proposed daytime standards, they will be deferred for up to a year pending a study, according to the source.

The plan also had called for tightening the noise limits in 1986 to force airlines to use a new generation of quieter jets now under development. That date is now subject to change pending another study, as there are serious questions, the source said, over whether the industry has sufficient money and production capacity to change to the new technology that soon.

The bill's $10.4 billion for the Transportation Department and related agencies in fiscal 1982 represents a $2.4 billion reduction from the fiscal year 1981 appropriation.An amendment to reduce funding by a further $323 million, apparently inspired by administration calls for a 12 percent cut, was defeated 39 to 52.

As passed, the bill would provide $290 million to Metro for construction and equipment purchase for the current fiscal year, $25 million less than the transit system's building program assumes. But Metro officials say that they could accommodate a cut that size without major disruption.

The bill could place further financial pressure on Metro in the area of operating assistance by reducing nationwide operating aid outlays slightly from last year's levels, and calls for the use of 1980 census results in allocating the money, which could mean more cities competing for the diminished funds would qualify for the funds. Federal aid current covers about 8 percent of Metro's operating costs.

All the bill's figures are subject to final agreement with the House.