Thousands of top government man-managers would be asked to join a tough new elite corps -- giving up job tenure for opportunities to move up faster -- under a civil service "reform" plan President Carter will unveil today at noon.

Under the reform package, part of the long-awaited White House plan to streamline personnel practices for the government's 2.6 million workers, the President will ask Congress to approve a separate pay and job system for "managers" in Grades 16 through 18.

Under the proposed Senior Executive Service, both career and political managers in the $42,000 to $47,500 pay range would be taken from the regular civil service system, in which tenure is a way of life.

Present "supergrade" managers would not have to join the SES, although in declining they could assign themselves to a no-promotion status.Persons entering grades 16 and above in future as "managers" would be required to join the SES. Agency heads would have greater say in what they were paid, what they did and where they worked. And SES members could be bounced back to lower level jobs in the career service if they did not work out, or refused to take assignments or transfers.

Earlier, the Carter team had talked about making all GS 16 through 18 employees -- nearly 10,000 in all -- join the SES. But pressure from in-house advisers persuaded them to limit the SES to "managers" and leave other specialists, scientists and technicians in the regular civil service system.

Supergrade employes in most agencies would be covered by the SES. Only those in the CIA, Tennessee Valley Authority and Foreign Service would remain outside it.

Many of the people going into the SES -- if Congress approves it -- would come from the Washington area. This area is loaded with top-graded workers who supervise headquarters activities employing more than 350,000 civil servants.

Persons who join the SES could be removed from their jobs "at the pleasure of the appointing authority," usually the head of the agency. Unless removed for gross incompetence or criminal actions, they would be guaranteed "retreat rights" back to Grade 15 level jobs in the regular civil service.

Veterans preference would not apply to the SES. And regular reassignments within agencies and to other federal departments and jobs would be a basic feature of the program.

Pay for executives entering the SES would be set by agency heads based on the ability of the worker, within the present range for Grade 16 through 18 which is $42,423 to $47,500. The Carter administration earlier wanted to be able to offer bonuses of up to 20 percent each year to top executives, but it could not be learned if this will be part of the president's recommendations today.

Federal employes who want to hear details of the president's plan -- which also calls for a speed-up and steamlining of hiring, disciplinary and firing actions for rank-and-file workers, can tune in to his National Press Club talk scheduled to begin at noon. WETA-FM radio will carry the talk live. A briefing on the "reform" package will be given later today to reporters.

FBI Jobs: The FBI has a lot of job openings at headquarters here for clerks, typists and stenographers. The jobs start at from $7.035 to $8.902. Seniors about to graduate from high school are welcome to apply, the bureau says. Call 252-7960 for information.

Where The Jobs Are: People who are hunting jobs with Uncle Sam in metropolitan Washington are often stunned to learn that the government has more than 200 employment offices here.

An outfit called the Federal Employment Directory Service has managed to track down most of those hiring offices and (for $5.50) will mail a copy of its updated, 30-page directory to you. Address of the FED is P.O. Box 34740, Bethesda, Md. 20034.

Engineering Jobs: Fort Belvoir has openings for Grade 9 through 12 civil and electrical engineers; a GS 12 general engineer, GS 5 mechanical engineers and an interdisciplinary engineer. Call 664-6467 for details.