Rapid economic growth still stumps experts

''The economy is going a little bananas,'' says Jerry Jasinowski, chief economist and executive vice-president of the National Association of Manufacturers.

That sentiment was widely shared among economists after the Commerce Department said Monday that the gross national product (GNP) shot ahead at an unexpectedly strong 7.5 percent annual rate in the April to June period (after adjustment for inflation and seasonal factors). The GNP is the value of goods and services produced in the economy. At the same time, the government revised from 9.7 percent to 10.1 percent its estimate of the pace of real economic growth in the first quarter of 1984.

''What is going on now smacks of a boom. There is no other way to describe the very broad-based strength of the economy,'' adds Allen Sinai, chief economist at Shearson Lehman/American Express.

''The strength of the recovery has almost consistently fooled everybody,'' says Commerce Secretary Malcolm Baldrige. The government's ''flash'' GNP estimate for the second quarter, issued last month, underestimated the recovery's pace by 1.8 percentage points.

As a result of the ongoing surge in the economy, the current expansion now has produced the second largest gain in GNP of any post-World War II recovery. In the last six quarters the economy has grown 10.9 percent. The current recovery began in the fourth quarter of 1982. The only stronger recovery began in the fourth quarter of 1949. Six quarters into that recovery the economy had surged ahead 17.3 percent.

Meanwhile, inflation, as calculated by a GNP-related measure called the implicit price deflator, ran at a 3.2 percent rate in the second quarter of 1984 , down from the 4.4 percent rate posted in the first three months of 1984. The more modest inflation rate was largely due to food prices, which dropped in the second quarter after rising sharply in the first.

''I don't know how you could write a script for a better recovery than you have right now,'' Secretary Baldrige told reporters.

The latest report on the economy appears to be good news for President Reagan , who is seeking reelection at least partly on the basis of his management of the economy. White House spokesman Larry Speakes greeted the economic news enthusiastically. Rapid economic growth is likely to create additional jobs and thus make further reductions in unemployment.

But some economists warn that the fast growth is bringing the economy closer to full use of its productive capacity. In general, the closer an economy gets to full use of its factories and labor force, the greater the upward pressure on prices.

''Seven-and-a-half percent growth in the sixth quarter of a recovery means you are growing twice as fast as your capacity to produce'' is expanding, says Donald Ratajczak, director of the Economic Forecasting Project at Georgia State University. ''We are speeding toward a wall and getting close to it. If we don't slow down, we'll get an inflationary crash.''

Economists are divided on whether the fast pace of economic growth will prompt the Federal Reserve Board to tighten credit conditions in an bid to head off inflation. The growth rates for the first and second quarters ''are in excess of what is desired by the Fed,'' Shearson's Mr. Sinai says. On Friday, the Fed announced it had not moved to tighten credit conditions at the May meeting of its policy setting committee. The minutes of those meetings are released on a delayed basis.

Even without specific Fed action to tighten credit conditions, surging economic growth puts upward pressure on interest rates as consumers and corporations seek loans, says Bernard Markstein III, senior economist at Chase Econometrics.

Secretary Baldrige argued that inflationary and interest-rate concerns are groundless. He sees the economy slowing to a 4 to 5.5 percent growth rate in the second half ''less than 5 percent'' in the fourth quarter. The trend in interest rates ''will be down by the year's end,'' he said. A lot of executives and economists ''are still worrying about yesterday's problem'' of inflation, he added.

One sign that economic growth may be about to moderate, Baldrige said, is growth in the portion of disposable income that individuals are saving. The savings rate jumped from 5.3 percent in the fourth quarter of 1983 to 6.1 percent in the first quarter of 1984 and 6.0 in the second. As a recovery matures, consumers cut back on purchases of big-ticket items and tend to save more of their income, thus slowing the pace of economic growth. But if it continues, this increase in saving would also boost the supply of lendable funds , thus easing the upward pressure in interest rates.

Whether or not their concerns were justified, investors seemed worried by the GNP figures: In early trading Monday, the Dow Jones industrial average dropped sharply.

The economy's second-quarter strength reflected a huge 10.4 percent increase in final sales. About a fourth of this increase was offset by a decline in the growth of business inventories. Part of the decline in inventory building was the result of farmers no longer getting crops from the government under the payment-in-kind program. Business investment in the second quarter grew at a 20. 7 percent seasonally adjusted annual rate. Capital spending in the past six quarters has been stronger than in any previous post-World War II recovery.

Among the key reasons for strong economic growth are the 1981 tax cuts and the ''enormously stimulative'' federal budget deficit, says Robert Gough, an economist with Data Resources Inc., a forecasting firm.