morse

code

communication

covert

message

signal

signaling

signalling

encoded

espionage

secret

telecommunications

transmission

data stream

data transfer

transmitting

text

on-off tones

dits and dahs

dots and dashes

synth

military sfx

underground

suspense

electronic

ambience

ambiance

background

telegraph machine

telegraphy

spy

spies

electrical device

cipher

sequence

communicate

underlying

counterintelligence

intelligence

agent

secrecy

concealed

cinematic

film

movie

scene

documentary

trailer

sound effect