A rumor floats about, as rumors will in Washington, on the latest in potential casting from the administration that gave you a movie star in the White House, picked another for the U.S. Embassy in Mexico City and found cameo roles in the halls in government for a film industry executive or two.

The name is Charlton Heston and the spot is head of the National Endowment for the Arts.

Is this rumor, which seems to have sprouted in the hills of Hollywood rather than along the Potomac, true? "We'd be the last ones to know," said a spokesman for the endowment, adding that the "normal procedure" is to set up a formal search committee well ahead of the expiration of the term of the current chairman.

Is that so? "We don't have any search committee," said an official in the White House personnel office, " but we are looking for people."

The White House said there is a list of candidates for chairman of the arts endowment, and Heston, who won an Oscar for his performance in "Ben Hur" and went on to star in an assortment of high-budget costume epics, would be on it, but it's understood that he would not be interested in doing the job full time.

After all, noted an endowment spokesman, "He's in the million-dollar-picture class."

He also has some familiarity with the turf. In addition to being a close friend of the president and -- like Reagan and ambassador-designate John Gavin -- a former president of the Screen Actors Guild, Heston has served a term on the National Council on the Arts, as a Lyndon B. Johnson appointee.

Meanwhile, the candidate lists for the arts agency and its sister, the National Endowment for the Humanities, already contain six to 10 names each, albeit names of lesser luster. But the choices won't be revealed until the terms of the current chairman are winding up. That's Sept. 14 for humanities Chairman Joseph Duffey and Nov. 6 for arts Chairman Livingston L. Biddle Jr.