FRANKFURT, West Germany (AP) _ The motto for intruders at U.S. military installations these days is: When you hear a goose, duck.

Army security forces are using geese, which object loudly when unaccustomed sounds offend their sensitive hearing, as an early-warning system at three sites in West Germany and expect to have them at about 30 installations beginning in September.

Goose Guards are new to the Army, but have a long tradition. The ancient Romans used them on the Palatine, one of Rome's seven hills. Legend has it that, during a siege by the Gauls in 390 B.C., the Palatine was saved by honking geese that warned the defenders of an attack.

The U.S. Army's 32nd Air Defense Command now has 18 warning honkers at installations of communications and radar equipment and anti-aircraft artillery.

''The command has tested geese at several sites, and they're still out there,'' Maj. Joe E. Padilla, spokesman for the command, told The Associated Press in a telephone interview Tuesday.

''They are intended as a natural warning system,'' he said. ''They make a lot of noise when they sense something strange nearby. That will alert our roving patrols, who can go to the area and check out what's happening.

''They won't replace the guards. They'll be there to alert them.''

Padilla said the idea of using geese came from the unit's commander, Maj. Gen. Victor J. Hugo Jr.

''He spent some time in the Special Forces and they are known for their innovative methods,'' the major said. Special Forces is the Army term for several highly trained specialty units, including the Green Berets.

Hugo sent his aide, Capt. David Thomas, to Scotland to observe Goose Guards used at the Ballantine whisky distillery after seeing an Armed Forces Network television program about it, Padilla said.

According to the major, Ballantine began using the geese in 1959, and even has erected signs warning intruders of possible attacks by the feathered guards.

After Thomas' visit to Scotland, the 32nd bought 18 geese to test at three locations, ''and they're an effective warning system, so we contracted for 900 more,'' Padilla said.

No one has been apprehended yet because of a geese warning, Padilla said. ''We can't prove that anyone was scared away by the geese, but they make so much noise it certainly is a possibility,'' he said.

All sites scheduled to receive Goose Guards are in central West Germany.

The spokesman said the Army will buy 750 female geese and 150 males at a cost of ''about $25,000 ... much cheaper than guard dogs.''

Tuesday's edition of the U.S. military's unofficial newspaper Stars and Stripes said the Army had arranged with West German farmers to raise the 900 ordinary white geese until they are nine months old.

Padilla said the geese are easy to take care of and no problem to feed because ''they eat what's on the ground, grass and things like that.''

Who will clean up after them?

''We'll probably just call that part of regular security maintenance.''