The federal hiring freeze is about to end.

Agencies that have reduced incoming traffic by 25 per cent (filling only three of every four vacancies) should be able to resume hiring after they get letters from the Office of Management and Budget spelling out their new personnel ceilings.

The limited freeze, imposed in early March, has kept many of the legion of federal job hunters (9,000 applications a day here) from joining the government payroll. For employees already on the job, the freeze has blocked a number of promotions.

President Carter ordered the 25 per cent hiring cutback to force agencies to trim down while they were awaiting new personnel ceilings. That review has been completed and the President has okayed new - generally lower - job levels for federal agencies.

Because of the freeze, most agencies will find themselves in good shape. Many will be able to hire normally, maybe beginning as early as next week.

The overall cut in federal employment is expected to be less than 2 per cent as a result of the Carter job ceilings. This will have virtually no impact on most agencies. It will result in a "paper" job cut of between 20,000 and 30,000 positions by President Ford in his final budget.

Insiders say that Defense wil take the major personnel reductions and that portions of Army, Navy and Air Force may have to continue the freeze longer. Defense accounts for nearly 50 per cent of the permanent full-time federal work force. There also is a chance that the National Aeronautice and Space Administration will have to continue the limited hiring freeze to help it conform to lower job ceilings.

The hiring freeze has had little effect on the typical civil servant. Some people have had to take on added duties to fill in for the "fourth person" slot left vacant because of the 3-for-4 hiring rule. Some employees have had promotions blocked because of the uncertain job picture.

Main throat of the hiring freeze has been on people trying to get on with Uncle Sam, and on some managers who haven't been able to fill all their job openings.

President Carter said he hoped to end the freeze by the end of April. But it lasted longer, in part at least because of Cabinet-level opposition to proposed lower job ceilings from the OMB. That opposition has either gone away, been satisfied or ignored and OMB now has letters ready for agencies advising them of their new ceilings. When they get them, in most cases they will be able to end the freeze.TDuring the month of March, the first full month of the limited hiring freeze, the number of full-time federal workers dropped by 1,224. Expert say that the attrition rate always drops during a freeze. This is because people who would normally be expected to retire hang onto their jobs during a freeze period, and employees who might have been planning to leave their agency for another government post or private industry tend to sit tight.

Ironically, once the hiring freeze is lifted, officials expect the number of employees who retire or quit will increase. That should open up a relatively supply of vacancies in agencies with favorable new job ceiling. Defense won't be one of those places, however.