High interest rates and a slumping economy pushed Tenneco Inc.'s third-quarter income down 4 percent to $156 million from $162 million a year earlier, with construction, farm equipment and chemicals operations affected most heavily, the company said yesterday.

Earnings per share dropped to $1.09 from $1.25 while operating revenues rose to $3.59 billion from $3.06 billion. Per-share earnings were based on an average of 128 million shares outstanding for the 1981 quarter compared with 117 million for the 1980 period.

Tenneco Chairman and Chief Executive Officer James L. Ketelsen said the firm's integrated oil, natural gas pipelines, shipbuilding and packaging divisions showed improved profitability for the quarter.

In energy, strong exploration and production results more than offset earnings declines in processing and marketing, Ketelsen said.

Nine-month net income rose 5 percent to $553 million on operating revenues of $11.17 billion from $526 million on operating revenues of $9.59 billion a year ago.

Earnings per share fell to $4.10 from $4.38, with the 1981 figure based on an average of 124 million shares outstanding versus 112 million for the 1980 period.

Braniff International Corp. yesterday reported a third-quarter net loss of $20.3 million compared with net income a year earlier of $17 million (85 cents a share). Net income in the third quarter of 1980 included a $61 million gain on the sale of aircraft, however.

The parent company of Braniff International Airways said it posted a third-quarter operating loss of $13.9 million. But that was 38.9 percent less than the third-quarter 1980 operating loss of $22.8 million, and $4 million less than it lost in the second quarter this year.

Braniff said its operating revenues for the third quarter of 1981 declined to $306.6 million from $381.1 million in the similar period of 1980.

Braniff Chairman John J. Casey blamed the lower revenues on the weakened economy and the air traffic controllers' strike. However, he praised what he called the "continuing support" of the airline's employes and lenders.

Braniff also reported a nine-month net loss of $85 million compared with a net loss of $53.5 million for the first three quarters of 1980. Operating revenues were $956.2 million compared with $1.127 billion the year before.