Senate Banking Committee Chairman William Proxmire (D-Wis.), in a letter to the comptroller general, has called for an investigation by the General Accounting Office of the lending policies of the Export-Import Bank.

The letter, dated Oct. 1, questions how long Ex-Im Bank can lend money to foreign borrowers at subsidized rates of about 8 percent a year.

If this policy continues, Proxmire says, "the bank will inevitably have to subsidize the difference between the low rate of return on these loans and the high cost of borrowed funds by using its earnings or dipping into its equity capital."

Ex-Im Bank, a federally funded institution, makes low-interest loans to foreign entities which use the funds to buy American-manufactured products. The idea is to permit U.S. exporters to match foreign competitors which often get subsidized financing from their governments.

Recently, the Carter administration has been tightening up on its liberal lending policy for the bank.

As a result of that policy of low-interest loans by Ex-Im Bank, the bank is committed to some $12 billion in loans to foreign borrowers, but it has an annual direct lending budget of only about $4 billion.

In August, with its 1980 budget already spent, Congress agreed to let the bank go to the costly private money market to fulfill $1.1 billion in lending commitments.

Proxmire, who didn't question the $1.1 billion request by the Ex-Im Bank in August, apparently is having second thoughts.