The East River and the soaring rectangle of the United Nations building frame the deceptively peaceful view from the eleventh-story window of Ambassador Andrew Young in the U.S. mission here.

But inside there is a sense of siege. The rockets of criticism are landing here form launching pads 250 miles away in Washington and elsewhere.

New political sobriquets are heaped upon Young - an apostle of "The Movement" of the 1960s who turned 45 yesterday - almost as fast as he breaks into the headlines: the Moynihan of the Left, the loose cannon, the wayward missile of the Carter administration, the "Andy Young Problem."

Within the ranks of the foreign policy bureaucracy, anonymous grumbles have been voiced against Young, charges that he is making a shambles of the precise and finely nuanced needlepoint of diplomacy, particularly in southern Africa.

The supposed gaffes by Young that have drawn fire now verge on political legend - headlines over Young's reported views on the stabilizing role of Cubans in Angola, on Henry Kissinger and the British role in southern Africa, on normalization of U.S. relations with Vietnam.

Each time Andy Young spoke, it seemed, the phones would ring or the teletypes chatter here and, boom, another rocket would land. Each burst seemed to enshrine further Andy Young's perverse celebrity in the pantheon of Jimmy Carter's government.

And so inevitably the question had formed in the minds of the bureaucrats in Washington, the diplomats in the U.N. lounge and the press corps: How long will Andy Young be allowed to continue his act?

If there is any unhappiness with him at the top of the government, there is no evidence of it. Quite the opposite. President Carter will go to Young's U.N. turf on Thursday to deliver his first major foreign policy address.

One authoritative White House staffer whose job it is to know and reflect Jimmy Carter's thinking insists that Young's stock has never been higher in th Oval Office.

"Andy Young has gone into an absolute disaster area for American foreign policy - sub-Saharan Africa - and it appears to me that we've finally begun to show some motion," he said.

"The ironic thing is that the folks who helped design that disaster, which Andy found in his lap, were taking potshots at him while he was over there trying to wrestle and struggle with the problem.

"This is not only unfair but also dumb, because it indicates a profound misunderstanding about the way the President reacts to things like that. The net result has been that with every little snotty shot from anonymous folks over at State, their influence and esteem has declined and Andy's has gone up."

Regarding the role of Secretary of State Cyrus R. Vance, the White House official said, "I would be very surprised if Cy Vance has done anything to undercut Andy. He finds those sort of shots as distasteful as the rest of us."

Another official with direct responsibility for foreign affairs - the ground zero of the Andy Young controversy - spoke of the UN ambassador's visit to Nigeria as "a sensational win for us. Remember, Kissinger couldn't set foot there."

He added: "If you are going to have an open administration you need a guy like this around, even if it causes problems. You want someone with his point of view and his constituency inside rather than out."

Another White House adviser observed: "The stuff Young is saying Carter probably agrees with."

Young looks out at the fray with unfailing imperturbability, with the confidence of a man President Carter described publicly as "the finest elected official that I've ever known."

"You've got a lot of nervous people who don't like our policies," Young said during an interview Wednesday. "Our policies say those people have been wrong. That's what the election was all about. That's what the American people confirmed in the response they gave President Carter."

Young acknowledges, as do his closest advisers, that he had made mistakes.

On Wednesday when his deputy, former missionary Brady Tyson, drew a mild presidential rebuke for making an extemporaneous apology at a U.N. meeting in Geneva for U.S. actions in Chile, Young ruminated:

"Brady won't make that mistake again. But that is probably not true of me. I'll probably make more mistakes because I'm more exposed."

At the White House on Thursday, press secretary Jody Powell sought to defuse the Tyson controversy with a touch of humor.

When a questioner recalled Carter's own campaign charges against the Nixon administration for the overthrow of Salvador Allende in Chile, Powell smilingly observed that Tyson was due "some degree of charity and understanding" for his Geneva statement.

Vance, who sat in the Pentagon during the 1960s while Young marched in the streets, has given no hint of a loss of personal confidence in Young despite several corrective responses issued in his name to comments by the U.N. ambassador.

During an airborne, off-the-record session with reporters in the course of his recent Middle East trip, Vance had nothing but the kindest words for Young, who shares Cabinet rank with him. "It was treacle," said one journalist of Vance's remarks.

The Andy Young story is tailor-made for a capital that adores controversy, pounces at the seem of political blood in the water and rises to the spectacle of someone flouring the conventions of prudent public utterance.

But it is also, as many administration insiders see it, a story of the generational change in style and values embodied in the Washington of Jimmy Carter. Young, a street missionary and disciple of Martin Luther King Jr., identifies himself as an advocate of the new political styles and values stemming from the racial and ideological conflicts of the '60s. And so when he opens his mouth, in in-baskets tremble in Foggy Bottom.

When he first discussed the U.N. job with Vance, young recalled, "I mentioned that there were a number of things the American people were thinking about, I told him that, if he did not mind, I would raise controversial points and talk about them. I said to Cy, 'You can refute and modify what I say, but I'd like to be your point man.'

"I told him that Vietnam was the one thing I would like not to have to modify my views on Martin Luther King's life was deeply imbued with concern over what Vietnam was doing to us. It would be a repudiation of him if I did not continue to take a position."

The role of point man, the lead position in a military patrol, is one which Young insists "goes with this job." This view affronts diplomatic conventional wisdom, which holds that an ambassador's job is to carry out his instructions from Washington and not stray beyond the reservation of his official brief.

But the role Carter created for Young is not cut from the cloth of conventional diplomacy.He sees himself as a catalyst for change, helping prepare public opinion for new policies by talking about things that have not yet been decided in the bureaucracy.

When Young predicted during his confirmation hearing that steps would be taken within 90 days toward normalizing relations with Vietnam, there was an uproar at the upper levels of the State Department. Yet, aside from his referece to the 90-day timetable, Young had said little more about Vietnam approchement than did Vance at his earlier conformation hearing.

As it turned out, Young was right. Well within the 90-day period Vance had appointed a mission headed by labor union leader Leonard Woodcock which leaves for Vietnam today to discuss the politically save topic of a final accounting for Americans missing in action and the far more explosive subject of the first steps toward re-establishing relations.

The Vietnamese last week invited a group of American journalists to Hanoi.It's not ping pong, but something is in track on Vietnam.

"It takes time for policies to change and part of helping hose policies to change is encouraging the American people to talk about it," Young said.

When he made his statement about Cubans introducing an "element of stability and order" in Angola, another State Department flap ensued and the State Department press office bristled with what seemed to be a repudiation. "Neither Ambassador Young or the Secretary condone the presence of Cuban troops in Angola," said spokesman Frederick Z. Brown.

Looking back at the controversy, a high-ranking State Department official well backgrounded in African affairs said: "The fact is that without Cuban technicians in Angola it would be an awful mess. There would be more fighting and total chaos. Young's choice of words may have been unfortunate but on substance he was right."

Young and others in the administration feel that the traumas over his style and utterances stem in part from the contrast with Henry Kissinger's autocratic style of State Department goverance.

"When you have a State Department where for eight years only one man speaks and thinks for everyone, my kind of statement is a problem," said Young. "But it is a problem for the spokesman and a few foreign service officers who still have the mentality of the Kissinger State Department."

Young senses that the disagreements with his policies are strongest in the Bureau of African Affairs, where Vance retained most of the diplomatic team which was trying to carry out Kissinger's policies in southern Africa. Since South African leader John Vorster was regarded as critical under Kissinger doctrine to a peaceful transition in Rhodesia, there were muffled howls within the bureau at statements by Young on the White South African leadership. There was also criticism of Young for what one official described as "increasing the linkage between blacks in Detroit and Capetown."

Whatever the extent of the grumbling, Young's influence within the Carter administration is probably unrivalled by any predecessor in the U.N. job. In assigning him Cabinet rank, the President designated him as second only to Vance in the governmental foreign policy hierarchy.

Young will have an office and working staff on the seventh floor of the State Department, the level occupied by the Secretary of State. It is an important symbol in a building where status is calibrated by the length of the elevator ride. He sits in both Cabinet and National Security Council meetings or is represented by one of his two principal deputies, career diplomats James Leonard and Don McHenry.

This means that Young is in at the takeoff of foreign policy formulation, before "all the work has been done, the options all molded," as one aide put it.

If that sounds like bureaucratic abstraction, Young has a vivid memory of the rates of predecessors in the U.N. job. At his confirmation hearing Young was reminded by Sen. Frank Church (D-Idaho) of the "humiliation,"of the late Adlai Stevenson, who had been kept in the dark about the Bay of Pigs. Young also recalled the private despair of Arthur Goldberg over the Vietnam policies of Lyndon Johnson.

During his hearing Young was questioned by Sen. John C. Danforth (R-Mo.) about his willingness to serve as a "team player" at State even if it meant advocating a policy he disagreed with personally.

"I would think that if there were differences in principle," Young replied, tearing the canons of bureaucratic loyalty to shreds, "I would certainly not sacrifice my principles. I would rather submit to the President my resignation with no hard feelings, and I would expect him to accept it . . .'

That is Young's view of how it is, out at the point.