Unison Whiteman, the foreign minister of the Caribbean island nation of Grenada, has been in Washington almost a week now on what he would like to think is an official visit. He has been to Capitol Hill to talk with the Black Caucus and other congressmen and he has met with the press. He even arranged a chat with the visiting president of Venezuela.

But Whiteman, whose leftist-ruled country of 110,000 people is little more than a speck on the Caribbean map, says he has yet to meet with a single official of the Reagan administration, despite his requests and the nominal diplomatic relations between the United States and Grenada.

It is not that the State Department is disinterested -- if anything, the Grenadans say, the United States is paying too much attention to their island these days. Rather, in what appears to be one of the more unusual manifestations of President Reagan's Latin American policy, the administration is attempting to show its displeasure over Grenada's close ties to Cuba by simply refusing to communicate with anyone in the Grenadan government.

The Grenadans say, in fact, that the White House will not even answer their letters. Prime Minister Maurice Bishop has sent two letters to President Reagan this year in an effort to seek better relations. The first, in March, prompted no response; the second, in August, won a one-paragraph reply from the U.S. ambassador responsible for the small nations in the eastern Caribbean saying only that the administration would "continue to look for serious indications" by Grenada of a desire for improved ties.

This unnerving silence, combined with a series of U.S. efforts to stop Western development aid to Grenada and a practice invasion of an island suspiciously similar to Grenada during U.S. military maneuvers this fall, has produced a predictable effect on the country's leftist leaders.

Not only does Whiteman accuse the United States, "a massive and powerful country," of waging "a campaign of economic aggression and strangulation" against "a small, poor country trying to break out of a cycle of poverty," but he and other Grenadan officials say they are convinced that the United States is planning a military invasion.

The State Department denies that charge, along with any intent to "strangle" Grenada. But when asked about the refusal to meet with Whiteman or the unanswered calls and letters, State Department spokesmen refuse all comment -- a position in keeping with the silent treatment being accorded a 22-mile-wide-island that, U.S. officials will privately charge, is a potential staging point for Cuban planes and troops and a general menace to U.S. interests.

The quiet intimidation policy for Grenada is not without precedent. The Grenadan goverment was treated similarly during the last year of the Carter administration, which told the U.S. ambassador in the eastern Caribbean not to visit the island while rejecting Grenada's candidate for ambassador to the United States.

But the Grenadans and diplomatic observers in Washington note that the relations freeze has been implemented more forcefully in the past year, and this concerns some congressional critics and U.S. allies, who feel Washington's hostility is only driving the Grenadans closer to Cuba.

Rep. Michael Barnes (D-Md.), chairman of the House Foreign Affairs subcommittee on inter-American affairs, said after meeting with Whiteman this week that the administration policy was "short-sighted." And Venezuelan President Luis Herrera Campins, whose country lies just south of Grenada and whose oil tankers are often mentioned by U.S. officials in Grenada-threat scenarios, noted in an interview that he had met with Whiteman during his stay in Washington, and added, "We are trying to make it possible that the new nations don't feel distanced from the democratic governments of the hemisphere."

The Grenadans vehemently deny a military connection to Cuba, and publicly have ridiculed the U.S. perceptions -- apparently based largely on a airport runway the Cubans are helping to build. "Obviously," Whiteman told a group of reporters this week with an appropriate look of bewilderment, "we are not a threat to the United States."

U.S. officials have contended that the runway is longer than needed for the tourist trade, and would be convenient as a fueling stop for military traffic to and from Cuba, 1,000 miles to the northwest.

But Grenada says the airport has no such future, and although the 2 1/2-year-old government has never hesitated to launch verbal attacks on the United States, it has been trying at least outwardly all year to establish regular communications with the White House and State Department -- to no avail.

Since January, for example, Grenada has been trying to get the administration to accept or reject its latest candidate for ambassador to Washington. The answer has been dead silence. In fact, the Grenadans and other observers report that in addition to the unanswered letters, the State Department official in charge of monitoring the eastern Caribbean has been instructed not to answer calls from the Grenadan representative now stationed in town.

Meanwhile, the United States has proposed a new ambassador for the group of Caribbean islands including Grenada, but excluded Grenada from his charge, and offered funds to the Caribbean Development Bank on the condition that Grenada not receive any. When Grenada sought loans from the International Monetary Fund and the European Economic Community this year, Washington aggressively lobbied against them.

And when U.S. naval and marine forces were on maneuvers in the Caribbean last August, they staged a mock invasion of an island off the Puerto Rico dubbed "Amber in the Amberdines," a label uncomfortably close for the Grenadans to Grenada of the Grenadines, which has an area on its southern tip called Amber.

Whiteman, who still maintains that Grenada would like to resolve its problems with the United States, professes to be puzzled about the motives behind all the snubs. "I wish I knew the reasons" behind the U.S. perception of tiny Grenada's threat, he said. "If we could get talks going, that would be the first item on the agenda."