In late 80's I read a short story
"The Sound" by A. E. van Vogt, a novelette in his Rull series; first published in Astounding Science Fiction, February 1950, available at the Internet Archive.
where a boy goes out night searching for "the sound" along with other children.
"It's me, George." The woman whose face grew onto the videoplate looked agitated. "George, the Play Square just called me. Diddy has gone out to look for the sound."
"Oh," said George.
[. . . .]
"Veda," he said sharply, "You're not letting it get you."
"But he's out there. And the whole area is said to be full of Yevd spies." She shuddered as she spoke the name of the great enemy.
"The Play Square let him go, didn't it? It must think he's ready."
"But he'll be out all night."
Craig nodded slowly. "Look, darling, this had to happen. It's part of growing up, and we've been expecting it since his ninth birthday last May."
A secure building or buildings were only accessible by real children because some sort of air hypodermic needle barriers would inject something into your blood, which would kill the aliens. . . . The alien 'children' want a sample of a child's blood to detect the poison and make an antidote so they can enter.
"Hold out your hand."
He held it out, tense and scared. I'm going to die, he thought. And he had to fight back the tears. But his training won out, and he stood still as a needle-sharp pain jabbed his finger.
"Just taking a sample of your blood, kid. You see, the way we look at it, that suction system out there conceals high-powered air hypodermics, which send up bacteria to which the Yevd are vulnerable. Naturally, these air hypodermics send up their shots of bacteria at about a thousand miles an hour, so fast that they penetrate your skin without you feeling them or leaving a mark. And the reason the suction ventilators keep pulling in so much air is to prevent the bacteria from escaping into the atmosphere. And also the same culture of bacteria is probably used over and over again. You see where that leads us?"
Various adults aid the boy along the way.
The woman went on swiftly. "Slip the gun into your pocket. Don’t use it until you’re ordered to. Good luck."
The aliens also cannot hear whispers.
“Diddy"—it was a whisper from above and to one side of him—"don’t show any sign that you hear this." Diddy stiffened, then quickly relaxed. It had been proved long ago that the Yevd electronic hearing and talking devices, located as they were inside sound deadening shoulder muscles, could not detect whispers.
The boy is given a gun and told all the real children are home safe, so all the "children" he sees are aliens and should be killed.
"Did you get them?" The whisper again.
"Yes." His voice trembled. Across the room what had been two apple-cheeked boys was changing. In death, the images couldn't hold. And though he had seen pictures of what was emerging, it was different seeing the dark flesh coming into view, the strange legs—
"Listen—" the whisper caught him out of that shock—"all the doors are locked. Nobody can get in, nobody out. Start walking through the building. Shoot everybody you see. Everybody! Accept no pleas, no pretense that they are just kids. We've kept track of every other real boy, and there are only Yevd in the building. Burn them all without mercy."
He arrives home and night and sleeps soundly.
He slept, feeling the pulse of the sound deep inside him, a part of his life.
Here is the Wikipedia plot summary:
After many years of warfare with the alien Yevd, mankind is on the verge of completing The Ship. The Yevd are aware of this, and have tried to infiltrate the shipyard in Solar City where it is being built. Humans have kept the Yved out by spraying tailored microbes into the air around the yard.
The Yved use light to communicate, and these organs can also be used to cloak their bodies in a shapeshifting fashion, or when applied in a burst, used as a weapon. As they do not use sound for communication they lack anything equivalent to ears, so to communicate with humans they use small translator boxes that are not very sensitive and cannot pick up whispers.
Diddy, a nine-year-old boy, is on his first hunt for The Sound, a mysterious humming sound that can be heard for miles around the yards. Finding the source of The Sound has become a sort of treasure hunt for young boys. Diddy is younger than most for his first hunt.
As he begins to explore the yard he is approached by a policeman that he immediately realizes is a Yved spy. The Yved tells him that that he is worried that the bacterial defense system has become stale and will no longer work. He asks Diddy to cross into the yard and then return. When he does, the policeman takes a sample of his blood. After a quick test, the Yved cross into the yard. Several more join Diddy, pretending to also be on the hunt.
As they explore the yard, they are approached by a young woman. She offers to tell them each a secret that will help them find The Sound. She whispers to Diddy that they have a plan to attack Yved. She tells him there is a gun hidden below a girder in a nearby building that he should retrieve while pretending to look for The Sound. He retrieves the gun and is then given further instructions through a low-volume speaker.
It is explained the Yved cannot use their light communications system in the presence of fluorine, and that they have filled nearby buildings with this gas. Diddy is told to enter another nearby building and shoot everyone, they are all Yved spies. Diddy enters and begins shooting, and when the Yved attempt to return fire, they burst into flame. He moves from room to room until the building is cleared.
With the threat ended, Diddy is allowed to leave. He joins a crowd of other boys watching the sun rise after a night of hunting for The Sound. Discussing it with another boy, Mart, they both realize they have found The Sound, it is the all-pervasive siren song of the entire shipyard put together.