The scream of an old civil defense siren -- loud, monotonous and a little troubling -- sounded unexpectedly through a substantial section of Northwest Washington last night, prompting dozens of calls to authorities.

Police said they traced the mysterious noise to a malfunction in a siren installed at Francis Junior High School at 24th and N streets NW and shut it off.

But for about 20 minutes starting about 8:30 p.m., the cry of the siren provided a cause for curiosity and some concern among residents of a city attuned to broadcasts from the the Persian Gulf war zone, where sirens signify air attacks.

Queries about the siren came from neighborhoods that included Georgetown, Foggy Bottom, Adams-Morgan and Mount Pleasant, some of them from people living as far as two miles from the source.

"We got millions of 'em," a sergeant at the 3rd Police District said of the calls. "They didn't know. We don't know . . . . They were curious."

In addition to being merely curious, some callers "were certainly concerned," said Denise Nash, a public affairs specialist with the city's Department of Emergency Preparedness.

"It just seemed uncanny that it would go off in this particular time," she said.

"There are people for whom this is setting off bad memories," said a woman who telephoned The Washington Post from Connecticut Avenue NW near the National Zoo. "The last thing we need to hear before going to bed is an air raid siren."