Low frequency pulsing and humming signal:
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White noise
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Geiger counter
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Interference Broadcasting
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Low frequency electronic signal
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Morse code, welcome
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Low frequency pulsing and humming signal
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Electronic signal, energy buildup
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Morse code, reception, transfer
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Morse code transmission
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SOS signal reception, Morse code
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Police radio
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Horrifying throbbing low-frequency rumble
★★★★★