The United States ambassador to Moscow, Malcolm Toon, once a strong supporter of the newly-signed strategic arms limitation treaty, has hinted privately he now has grave reservations about verification and may not endorse the pact.

News of the blunt-spoken career diplomat's possible change of heart has moved quickly through the American diplomatic community since the Vienna summit, which ended Monday. One source said Toon has conveyed his new views to Secretary of State Cyrus Vance's staff while the two were in the Austrian capital.

The administration reportedly had at one time planned for Toon, a Republican who was first appointed as an envoy by former President Nixon, to speak out publicly at a series of engagements on behalf of the treaty after his long-planned retirement this year. His possible defection could be a blow to the White House in the fight for Senate ratification.

Toon has told some aides that he hopes to be able to resolve his reservations favorably during consultations in Washington next month.

At the same time, Toon is said to have sharply questioned the Carter administration's choice of retired IBM chairman Thomas J. Watson Jr., a multi-millionaire businessman with no experience either as a diplomat or in dealing with the Soviets, to succeed Toon when he retires later this summer.

Sources said Toon earlier made clear to the administration his conviction that the post should be in the hands of a veteran diplomat who knew how to deal with the Soviets, whom he views as resourceful, tough minded opportunists who will have little trouble taking advantage of a novice ambassador.

It is understood that Toon's reservations about the SALT pact center on the loss of surveillance posts in Iran, from which the United States earlier intercepted telemetry data from Soviet test missiles launched in Central Asia north of the Caspian Sea. That intelligence-gathering operation was halted by the Iranian revolution, and the United States has not yet found a way to replace it.

Administration attempts to gain permission to use Turkey's air space for U-2 spy planes to attempt the job once done from ground stations in Iran have so far yielded nothing. The Turks have said they will consult first with their Soviet neighbors.

Since the ambassador's reservations reportedly center on adequate replacement for the lost Iranian posts, it is unclear precisely what caused Toon to begin pulling back from all-out support now instead of several months earlier after the Iranian revolution.

The question of treaty verification has become the focal point of the SALT II opponents, and troubles many senators who have not yet publicly announced their intentions.

Toon's new questions arise just ten days before the visit here of Senate Majority Leader Robert Byrd (D-W. Va.) on a private, four-day tour in which he reportedly hopes to meet with Soviet leader Leonid Brezhnev. Byrd has not yet made up his mind on the treaty and the visit here could prove crucial to his decision, which in turn will influence a number of other senators.

Watson, 65, retired president and chairman of the board of the giant computer company, has been told he is the administrations choice, according to highly reliable American sources. But the reports have touched off opposition in Washington on grounds that Watson, with no direct experience in negotiating with the Soviets, would make an ineffective envoy. Although Soviet-American relations have improved since the summit, tensions are sure to be generated out of the SALT ratification fight in the Senate.

The administration is said to have sounded out Lawrence Eagleburger, former aide to secretary of state Henry Kissinger, who is now ambassador to Yugosavia, for the post, but was turned down. Harry Barnes, director general of the Foreign Service, was also considered for the job. Both men have experience dealing with the Soviets.

Toon, who turns 62 next month, has served here since December 1976. Initially a major figure on the American SALT negotiating team, he has had progressively less to do with the bagaining in the past year. CAPTION: Picture, Ambassador Toon was once a strong SALT supporter. AP