CONGRESS SEEMS to be having trouble finding further reductions in the budget. But President Reagan says that, one way or another, he needs another $16 billion worth of progress this year toward lower deficits. The time has come for another long, unsentimental look at the kind of expenditures that don't appear in the budget but count in the result--the tax expenditures.

A tax expenditure is the money that Congress chooses not to collect, for some specially defined reason, from the tax that you would otherwise have to pay. Most are useful and necessary. But there are several that, in this difficult moment, deserve attention once again.

DISC--the domestic international sales corporation--offers indefinite tax shelter to exporters' profits. It's a blatant export subsidy, and some of this country's trading partners have recently been complaining that it's illegal. At the least, it's bad policy and a waste of money, supporting many exports that are fully competitive without it. DISC is an expensive luxury, costing $1.6 billion a year.

There's that familiar target, percentage depletion of gas, oil and a variety of other minerals. It's been curtailed, but it ought to be eliminated entirely. To go to the more defensible principle of cost depletion would save nearly $3 billion this year.

It's hard to think of any good reason for the deduction for consumer credit interest costs. The money with which families buy groceries and pay the light bill is fully taxed. Why an exemption for the money that pays interest penalties on delinquent household bills? It costs the federal government $6 billion a year.

Congress has recently limited the exemption for unemployment compensation, but most of it is still untaxed. Why should one person's unemployment benefits be untaxed when another person with the same annual income, earned as wages, is fully taxed on it? The exemption costs $4.5 billion a year.

And then--delicate subject--there's the deduction for mortgage interest. It costs the government $25 billion a year. No one seriously proposes revoking it. But there's a strong case for putting a cap on it.

A totally rational tax policy would move toward much lower tax rates than the present ones, with much simpler rules and fewer special exemptions, credits and deductions for this and that. The trouble is that all those special exemptions, etc., are very popular for reasons that seem to go beyond mere money. A lot of taxpayers evidently like to match wits with the Internal Revenue Service, and see the complexity of the system as an opportunity to strike back against Darth Vader. The proliferation of special tax benefits has turned taxpaying into a sort of a scavenger hunt that has replaced baseball as the national game. But this season Congress might consider playing its defensive position with more vigor.