The Export-Import Bank, already under pressure from Reagan administration budget-cutters, faces federal investigations because of questions being raised about the terms of its federally subsidized loans to spur American exports -- terms that apparently come as a surprise to at least some of the bank's board of directors.

The concern, which has prompted a call from Sen. William Proxmire (D-Wis.) for a General Accounting Office investigation, is over new revelations about a huge loan to an Australian airline coowned by financier Rupert Murdoch. The $200 million loan, at an annual interest rate of 8 percent repayable over 10 years, was to finance purchase of five Boeing 767 jets by the airline, Ansett Transport Industries.

But terms of the loan contract, which have been made available to The Washington Post, indicate that more than 17 percent of the loan was to finance "credit memorandums" in excess of the cost of the planes, their installed engines and other items enumerated to the bank.

The document shows Ansett has a $34.2 million credit with Boeing and the General Electric Co., which manufactures the engines for the 767s. Of that amount, Boeing has credited Ansett with $16.5 million and GE has credited Ansett with $17.7 million.

The the case of the Boeing and GE credit memos, the bank apparently is financing a vaguely described list of items, such as advertising, publicity and parts, that will "support" Ansett's planes. But when the bank gave preliminary approval to the loan, several sources say, the officials in charge apparently thought the bank was financing the plane, the engines and specific other items, including pilot training, extra engines and ground support equipment, not credit memos.

The Ex-Im Bank loan technically is made to Ansett, which pays the interest, but the funds are paid to Boeing and GE over a period of time as they construct the planes. As a result, the sources say, the U.S. companies have use of the $34.2 million at no cost until Ansett uses the credit memo. If the two companies were to borrow the funds from a commercial bank, they would pay an annual interest rate of about 17 percent.

The existence of the credit memos apparently came as a surprise to at least some of the five members of the Ex-Im Bank board of directors who raised quesitons abaout terms of the loan in a meeting Feb. 24, at which final approval of the loan was discussed. Thus it appears that some members of the bank board, which lends billions of dollars to foreign buyers of U.S. goods, apparently have been unaware that tens of millions of dollars of the low-interest loans have gone to finance credit memos.

The Treasury Department, which reviews Ex-Im Bank loans over $30 million and had been in the dark about the credit memos until the Feb. 24 meeting, has demanded a full explanation from the bank.

Both Boeing and GE confirmed existence of the credit memos, calling them a commonplace marketing technique in airline sales, but neither would comment on the figures in the Ansett deal. A Boeing official described the figures as "highly proprietary" and called them "trade secrets."

The Carter administration backed the controversial loan by arguing that if the bank did not finance the planes at the subsidized rate of 8 percent, Boeing would lose the order to Airbus Industrie, a Eurpean consortium that manufactures the A300 passenger jet.

But some banking officials question the propriety of federally financing the memos at the low 8 percent annual rate.

According to sources, credit memos also figured in another recent Ex-Im Bank loan to finance Boeing planes sold to El Al Airlines of Israel. That loan was for $230.1 million and the credit memo from Boeing to El Al was $17 million, say three separate sources who have viewed the loan document.

The El Al and the Ansett loans have been referred to Congress, which has 25 days from referral date to raise objections. The Ansett loan was sent to Congress March 5, the El Al loan on March 12.

Proxmire, ranking minority member of the Senate Banking Committee, requested a GAO investigation Wednesday after receiving two anonymous calls concerning the credit memos, according to a staff member.

Proxmire asked the GAO to probe reports that "rebate practices not appearing in the public record of those transactions are in effect."

An internal Ex-Im Bank document, which was presented to the bank board at the Feb. 24 meeting and describes the Ansett loan, states that the credit memos are included in the 767 aircraft loan package. That document refers to a Jan. 18, 1979, letter to Boeing from Warren Glick, the bank's general counsel. In the letter, Glick told Boeing that credit memos can be funded by Ex-Im Bank loans, but they cannot be used to make cash rebates -- or kickbacks -- to airlines purchasing its airplanes.

Describing the current loan to Ansett, the document says, "By letter and cable Ansett has confirmed these payments will not be obtained in cash. Furthermore, Ansett has indicated that the credit memoranda will meet the criteria described in Ex-Im bank's Jan. 18, 1979, letter to the Boeing Co."

Sources close to the bank's board of directors says this document was the first indication to most of them that the bank was financing credit memos. Furthermore, these sources say, Glick apparently set the policy on credit memos in his Jan. 18 letter to Boeing without conferring with the board.

But Glick, in an interview, insisted the credit memos are not being financed by the bank. "Credit memos are over and above the contract price," he said, and added: "If they are not part of the contract, we don't see them. But we're not financing them either."

Glick also said "the practice of giving credit memos has been going on since 1945. We've never made a big point about it. The fact that the board is not keenly aware of them is not their fault.We didn't think they were of sufficient gravity, and we still don't.

Richard Albrecht, general counsel of Boeing, said the credit memos are to finance goods and services years in the future, when the planes are delivered.

"What is being financed is Ex-Im's appropriate share of what Ansett is paying. What (Ansett) is getting is the aircraft, plus whatever goods and services can be purchased with the credit memos," Albrecht said.

A spokesman for GE's Aircraft Engine Group in Evendale, Ohio, said simply: "GE has responded accurately and completely to a request for information from Ex-Im Bank regarding all credit arrangements granted to Ansett."