The nation's industrial production dropped 0.1 percent in September following a 0.6 percent rise in August, marking the second decline in the past three months.

Economists said the decline in industrial production -- which includes the output of factories, utilities and mines -- was not very significant and was caused by the large surge of automobile production in August resulting from car financing-rate discounts offered by the industry.

They said that because of the distortions caused by the flurry of automobile manufacturing in August, production should recover somewhat in October.

Meanwhile, two Reagan administration officials said yesterday that they expect the economy to bounce back in the second half of the year from the sluggish 1.1 percent rate of growth experienced in the first six months.

Commerce Secretary Malcolm Baldrige, addressing an American Stock Exchange Conference, said he expected that the government will announce today that growth in third-quarter gross national product -- the nation's output of goods and services -- was revised upward from the 2.8 percent rate estimated a month ago.

Baldrige, who heads the department that releases the GNP data, said he didn't know exactly what the figure will be, but that his analysis of the increases in the leading economic indicators, strong automobile sales and lower interest rates point toward an upward revision.

Because of the strong automobile production in August, economists said they expect the government to report that the nation's output in goods and services grew at about a 3 percent rate in the third quarter.

Beryl Sprinkel, chairman of the Council of Economic Advisers, said that the "odds are pretty good" that the administration's forecast of 5 percent growth in output for the second half of the year will be accurate.

Sprinkel, in remarks to reporters after a speech at the stock exchange conference, said that there were "more and more signs" that the economy will strengthen and continue to improve next year.

However, many private economists say that factory production will not improve more sharply unless U.S. exports are increased and domestic demand is served by domestic production.

The surge in automobile manufacturing during the summer has distorted other economic statistics, such as manufacturers' inventories and sales and retail sales, economists said.

"I expect a sharp bounce-back in industrial production in October," said David Jones, economist for Aubrey G. Lanston. "The number does not reflect the basic tone of the economy, which is stronger" than one would glean from the figures released yesterday, he said.

Jones said he expects third quarter growth to increase at a 3 percent rate, "and a pickup to about 4 percent in the fourth quarter as we see a combination of business inventory rebuilding from strong consumer demand, particularly from automobiles."

Jones said he also expects "less of a drag on the economy from the trade deficit. It will be at least stabilizing, rather than becoming larger." In addition, housing activity should be stronger in the fourth quarter than in the third, Jones said.

Industrial production in September was 1.1 percent above a year ago, compared with a 6.8 percent increase in production for 1984, the Federal Reserve Board said.

Production of manufacturers dropped 0.1 percent last month, led by a 0.3 percent drop among durable goods industries. Nondurable goods industries increased output by 0.2 percent.

Production of construction supplies rose for the seventh consecutive month, increasing 0.2 percent in September. Production of defense and space equipment rose 1 percent. Output of consumer goods fell 0.3 percent and production of cars and automotive products declined 2.1 percent.

Output at mines declined 0.7 percent in September, the third consecutive monthly decline. Output at utilities, however, rose 1.1 percent.