Late last month, President Reagan went before the annual meeting of the National Association of Manufacturers and proclaimed, "The economy has shown solid growth for the last nine quarters . . . creating new jobs at the rate of hundreds of thousands each month."

The president pointed with pride to a recovery in business investment, which he said "has been the strongest in some three decades," to rising productivity and to an inflation rate that is "at the lowest level in more than a decade."

The president's assessment of his administration's economic performance comes at a time, however, when unprecedented trade and budget deficits and the latest round of indicators have clouded the economic outlook and raised new concerns about how long the economy will continue to expand.

Some senior Reagan administration officials and many private economists are expressing serious concern about the way the rising trade deficit is dragging down economic growth. There is worry, too, about the fact that the United States, with a new reliance on imported capital as well as goods, soon will face a different kind of debt problem: how to pay interest on all the foreign money.

But the public emphasis is still on the brightness of the outlook, as it was last week when the Commerce Department estimated that the gross national product, adjusted for inflation, is rising at a 3.1 percent rate this quarter.

Because of problems in manufacturing and mining, which are being pounded by a rising flood of imported goods, real GNP had increased at only a 2.2 percent annual rate during the last three of the nine quarters of which Reagan spoke. That was little more than half as fast as the administration had forecast.

The higher estimate for this quarter doesn't help the average very much. The first-quarter figure was revised downward from 0.7 to 0.3 percent, leaving the current level of economic activity 2.3 percent higher than it was a year ago.

At the same time, the slow growth has left the civilian unemployment rate stuck near 7.3 percent for almost a year. Employment has gone up an average of about 150,000 a month because of hiring in such industries as retailing and services, but so has the size of the labor force. In fact, some economists are surprised that the unemployment rate has not gone up.

Instead, the number of hours production and nonsupervisory employes are working at businesses other than farms has gone up slightly faster than output over the last year. And that means productivity, contrary to Reagan's assertion at the NAM meeting, has risen little if any since the second quarter of 1984.

In manufacturing itself, nearly 150,000 production workers -- more than 1 percent of such workers in the nation -- had lost their jobs in the three months prior to the president's speech. Total manufacturing output was no higher than it had been 10 months earlier, and it was going down, not up. Another 28,000 such jobs were lost in May.

The surge in business investment highlighted by the president in his speech appears to be tapering off rapidly. The latest survey of business investment intentions indicates that real spending on new plants and equipment will rise at a 4.4 percent annual rate in the third quarter of this year and 1 percent in the fourth. Some other types of investment spending, such as for office buildings, still are rising more strongly, even though there is already a glut of unused office space in most cities around the country.

More important, the big increase in business investment over the last two years appears unlikely to add as much to future levels of U.S. national income as the large totals would seem to imply.

First, the bulk of the growth occurred in spending for computers and business automobiles, according to research by Brookings Institution economist Barry Bosworth, which will be published next month in the Brookings Papers on Economic Affairs. Because neither computers nor autos have very long useful lives, they must be replaced fairly quickly. This contrasts with the investment boom of the 1960s, when a substantially larger share of the money flowed into types of goods with relatively long lives.

Second, and far more striking, however, is what will happen to the income from last year's surge in investment. Directly or indirectly, it will have to be used, almost entirely, to pay a return to foreigners who invested or loaned a net of roughly $100 billion in the United States in 1984.

Unpublished Commerce Department figures indicate that about three-fourths of last year's nonresidential investment went just to replace worn-out or obsolete plants and equipment. The remaining one-fourth, about $106 billion worth, represented the 1984 increase in the stock of capital assets owned by business. Those new capital assets will help produce a flow of goods and services that will provide a return to the owners of the assets.

In some cases, foreign investors helped pay for those assets directly by buying shares in U.S. companies, joining limited partnerships to build or buy real estate, by acquiring firms outright, or in a host of other ways, such as purchasing corporate bonds. In other cases, the foreigners bought U.S. government securities issued by the Treasury to help finance the federal budget deficit, which meant American-owned money that otherwise would have gone to finance the growing federal debt was available to finance private investment.

Whether the private investments pay a return to the foreign owners directly, or the Treasury taxes the income from the investments and uses it to pay interest to foreigners who hold government securities, almost all of the income from those new capital assets will flow abroad, not to Americans.

That is just one of the unprecedented and uncomfortable aspects of the current economic reality in the United States. "I have never seen anything like this," said a senior Federal Reserve official, who has followed the nation's economic ups and downs for 30 years. "I look for precedents and I find nothing but empty boxes. I am more uncertain about what will happen than I have ever been in my professional life."

The traditional statistics of consumer demand, business investment, government spending, credit and money growth, and so forth are no longer enough to provide a good sense of the future, the Fed official said. Such factors are still important, but the "dominant variable is the exchange rate," he declared.

Allen Sinai of Shearson Lehman Brothers described some of the complex web of relationships that has brought about this current situation this way:

"The twin deficits -- budget and trade -- and the emerging net debtor status of the United States are major issues. The long string of huge U.S. federal budget deficits, record trade deficits and increasing trade debt are intertwined.

"Large federal budget deficits, in the context of monetary-growth targeting by the Federal Reserve, stimulate economic growth and raise nominal and real interest rates. Higher interest rates strengthen the dollar by making investment opportunities in the United States more attractive . A stronger dollar holds down inflation and raises real interest rates. The dollar is further strengthened.

"With strong economic growth and a strong dollar, imports increase and the pace of exports decreases. A worsening trade deficit and an increased trade debt result.

"So long as the huge federal budget deficits remain, the process continues until the trade sector becomes so weak that economic growth slows, interest rates drop, the dollar declines and the process is reversed.

"However, with no intervention to tighten the budget, the eventual result can be a lopsided, unbalanced economy, with numerous problems -- including chronic federal budget deficits, an overvalued domestic currency, permanent erosion in the relative market shares of the basic industrial sector and industries."

And what does that add up to? "Stagnant economic growth, high unemployment, still too high inflation, high real interest rates, and a fallout of failures in beleaguered sectors, industries and financial institutions," Sinai responded.

Concerned about the condition of financial institutions, the value of the dollar and the plight of the manufacturing and agricultural sectors, the Federal Reserve has been pumping large amounts of money and credit into the economy. In the process, the central bank has had to ignore its targets for growth of the most closely watched measure of money, M1. It also has allowed total private-sector debt to rise about 14 percent in the last year, a rate more than double the 6.2 percent rise in current-dollar GNP -- that is, GNP not adjusted for inflation.

As real economic growth has slowed and the Fed has responded, interest rates have declined. For instance, the prime lending rate at commercial banks was cut last week from 10 to 9 1/2 percent, the lowest level in nearly seven years. However, many analysts believe that, if the economy should start to expand at a faster clip, rates would rise again.

Meanwhile, many rates remain quite high, particularly relative to current and expected inflation. Consumers regularly are paying 18 percent or higher rates on revolving charge accounts or unsecured bank and finance company loans. Rates on conventional 30-year, fixed-rate home mortgages are still about 12 1/2 or 13 percent in many parts of the country. AAA-rated corporations are paying nearly 11 percent on long-term bonds. Even the federal government, the most highly regarded borrower of all, is paying about 10 1/2 percent for long-term money.

Inflation currently is running at about a 4 percent rate, and most forecasters expect it to rise only gradually over the next few years. That means that real interest rates -- the difference between the rate actually paid and the inflation rate -- remain at historically high levels.

Meanwhile, the federal government continues to sop up money from the capital markets like a sponge. The budget deficit for fiscal 1985, which ends Sept. 30, will be more than $200 billion, probably about $206 billion. That means that $806 billion would have been added to the national debt in the five years of the Reagan presidency, pushing the total to about $1.825 trillion at the end of the year. Over the same period, the portion of the debt held by the public will have risen from $715 billion to about $1.500 trillion.

A portion of the federal deficit is being offset, in terms of impact on financial markets, by surpluses of $50 billion to $55 billion at the state and local government level. Most of those surpluses are the result of payments into pension funds for state and local government employes.

But taken together, the total government deficit will still amount to about 3.8 percent of GNP.

In some countries, such as Japan or West Germany, such a deficit would have little impact because households and businesses there save such a large share of their incomes. In the United States, however, where the savings share is much lower and apparently has been little affected by all the business and personal income tax cuts of the last few years, the combined government deficit will absorb about half of this year's net national saving in the United States, according to estimates by the Washington Post based on forecasts from Data Resources Inc.

Net national saving, which will equal about 6.9 percent of GNP this year, is total saving by households, businesses and governments, less the portion of current saving needed just to replace capital goods that are wearing out.

Once the federal government has financed its deficit, about 3.1 percent of GNP is left to support all net investment. Probably about 1.8 percentage points of that will flow into housing this year, and perhaps another 0.8 percentage point into business inventories.

Thus, only about one-half percent of GNP remains to finance net business investment in new plants, equipment and other capital assets. But present investment intentions suggest about 2.9 percent will be needed.

All those investment plans plus the government deficit add up to about 9.3 percent of GNP, while the nation as a whole is saving less than 7 percent. The difference must be obtained from foreign investors or, because the federal budget deficit will be financed come what may, it will show up as a reduced level of investment in housing, inventories or business fixed investment.

This is the squeeze the economy faces, a squeeze that was not part of the original Reagan economic plan four years ago. That plan included no recession such as occurred in 1981 and 1982. Instead, real GNP was to grow 4 to 5 percent a year through 1986. At the same time, inflation was to drop below 5 percent by 1986.

Because of the recession, inflation came down far faster than even the administration had projected. However, the recession also pulled down the level of GNP. Together, the recession and lower inflation meant that this year's current-dollar GNP will be about $600 billion lower that in the original Reagan forecast.

Along the way, the lower levels of GNP also meant less federal government revenue and bigger deficits. The bigger deficits compounded the difficulty of cutting federal spending as interest payments mounted on the national debt. In fiscal 1984, those payments were about $45 billion higher than called for in the first Reagan forecast.

Different economists give different weights to some of the factors behind the unprecedented economic situation facing the nation. And they differ substantially over the short-term outlook. Some, such as Alan Greenspan of Townsend-Greenspan & Co., think the second half of this year will see a resumption of economic growth at a 4 percent rate or better. But Greenspan then also thinks a new recession will occur by late 1986 as the business cycle becomes fully mature.

A recession, of course -- even a shallow one -- would cause federal budget deficits to soar. David A. Stockman, director of the Office of Management and Budget, has been warning that, if the economy continues to grow at its recently subdued pace, that alone could increase the fiscal 1988 deficit by $70 billion.

When the government reported that real GNP seems to be growing at a 3.1 percent rate this quarter and consumer prices rose at only 0.2 percent in May, White House spokesman Larry Speakes said that the figures "point to a renewal period of stable growth with low inflation. . . .We feel that, with the drop in the prime rate to 9.5 percent, along with the rise in the nation's money supply and a slowdown in inventory accumulation, GNP growth can continue at a healthy pace."

Commerce Secretary Malcolm Baldrige was even more upbeat. "The worst of the slowdown probably is behind us, and we should be back on a higher growth path by summer's end," he declared.

But the problems remain and they are sufficiently severe, in the opinion of Greenspan, an occasional private adviser to the administration, that his long-term forecast for the 1985-1990 period is for the economy to grow at an average rate of just 2 percent a year.