A growing number of Vietnamese fleeing to refugee havens here have left homes in Northern Vietnam to escape the Communist government under which they lived for many years.

Since April, according to Hong Kong officials, as many northerners as southerners have come by boat from Vietnam, in sharp contrast to previous months when the refugee population was overwhelmingly southern.

Moreover, the officials say, the northerners who come out no longer are mostly ethnic Chinese virtually expelled by the Hanoi government. About half of them are Vietnamese who escaped on their own to seek a new life elsewhere.

Both patterns represent a new trend in the Vietnamese refugee flight. In the past, the typical refugee had been an ethnic Chinese living in the south, unhappy under the new Communist authorities who took command four years ago and forced to abandon his former capitalist livelihood.

Now he is as apt to be a northerner as a southerner and to have lived for decades under Communist governments and even fought in their armies.

Two of the notherners - both ethnic Chinese - told their stores today to a delegation of nine American congressment who visited their camp as the first stop on a six-day fact-finding mission in Southeast Asia.

Both of them said they were forced to leave under threats of being taken to the "new economic zones" in the countryside.

The delegation, headed by Rep. Benjamin Rosenthal (D-N.Y.), heard briefings by both American and Hong Kong officials on the situation of refugees here. The visit comes at a time when the overall exodus by boat has diminished, either because of Vietnam's plege to restrict the flow or because of weather conditions.

In a separate interview, newcomer Mai Xuan Ba said he left Hanoi because of government repression and because economic problems were causing widespread suffering. Asked if others want to leave for the same reasons, he replied, "Many, many."

The dissatisfaction had grown substantially in recent years as food problems increased. He said people in Hanoi blame the government for permitting rice needed at home to be sent to Vietnam's new allies in Laos and Cambodia.

He also said many homes of suspected dissidents are checked regularly by government police looking for hidden weapons. There are frequent arrests, he added, and many disappear into "hospitals" from which they never return.

Other northern refugees interviewed in a Hong Kong camp called Kai Tak-North were of Chinese origin and told stories similar to the ethnic Chinese from the south. They were given a choice of fleeing after paying bribes or facing a new, uncertain life in the "new economic zones."

Tang Dao Kien, who lived in Haiphong, is typical.

"They told us that we are Chinese and therefore not suitable," Tang said. "And so we have to get out."

If he had not left, the authorities would have forced him into one of the "new economic zones" where, he said, he would probably die of starvation.

Others, like Mai, are Vietnamese who escaped secretly. He emphasized that the government is not pressing people like him to leave the country, only the ethnic Chinese. The Vietnamese who want to get out have to escape.

Mai said he had been a loyal Communist in his youth and even became a political officer in an Army battallion from 1945 to 1950, when the Vietnam were fighting the French Army.

He said he got a reputation as an anticommunist misfit in the 1950s and in the past 36 years has served four separate jail terms. Once, in the 1960s, he was in a Hanoi jail when U.S. bombers raided the city.

Hong Kong refugee officials are puzzled and a bit worried by the changing pattern that has brought many northerners into the camps here.

"So many of them don't really say why the they home," said one official familiar with hundreds of refugee cases. "A lot of them just say, "Well, all my friends were leaving and so I decided to go too.""

He also said this colony's government is concerned because the new pattern suggests that hundreds of thousands more refugees may seek havens here. So long as only ethnic Chinese were considered likely to come out, the problem seemed manageable, the official said. But if many thousands of northern Vietnamese begin coming out, he said, the numbers could become overwhelming.

A few of the northern refugees have told officials here that they fought in the North Vietnamese Army against American and South Vietnamese forces. In the early stages of the exodus there were many northern refugees with high technical skills, even computer technicians. Now the flow includes many laborers and factory workers, the officials say.

Rosenthal, at a news conference, praised the Hong Kong government for its humanitarian treatment of the refugees here. More than 67,000 are in United Nations-sponsored camps in Hong Kong or in a dockside entry camp.

Rosenthal said the committee expects to make some recommendations to both the House leadership and President Carter on its return to Washington next week.

The special committee, appointed by the House leaders, flew tonight to Bangkok for a look at camps in Thailand, which now has more than 200,000 refugees. The House members will also visit Indonesia and Malaysia, and will spend part of one day talking with Vietnamese governments officials in Hanoi. CAPTION: Picture, Reps. Benjamin Rosenthal (D-N.Y.) and Robert Drinan (D-Mass.), studying the refugee issue, find a language that younsters in a Hong Kong camp understand. AP