A federal grand jury in Alexandria has begun an investigation into the source of information reported by CBS-TV and The Washington Post about a financial investigation of secret Army units and their work with the Central Intelligence Agency, according to sources.

Historically, such investigations of leaks rarely reach the grand jury stage, sources said. This is due in part to the inability of the agency requesting the investigation to narrow the list of suspects and its reluctance to declassify the material for a trial, they said.

The most recent case -- and the first in which a government employe was convicted on espionage charges for giving classified material to the news media -- was that of Samuel Loring Morison. He was sentenced this week in Baltimore to two years in prison for having given secret U.S. satellite photos to a British defense magazine.

Leaks often come from high-ranking government officials trying to promote their policy objectives, studies have found. At times, highly classified information leaks inadvertently, such as when President Reagan recently stated in public that he favors a progran of covert aid to Angolan insurgents.

The Alexandria grand jury probe, supervised by Assistant U.S. Attorney Theodore Greenberg, began shortly after CBS-TV broadcast a report Nov. 22 about activities of a secret Army aviation unit that worked with the CIA on covert operations, sources said. The Post reported one week later on the Army's financial investigation of several of its special operations and intelligence units, including the aviation unit, known as Seaspray and operated out of Fort Eustis, Va.

Greenberg declined to comment on the investigation yesterday.