Low-frequency rumble, high-frequency squeak:
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Interference Broadcasting
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Low frequency pulsing and humming signal
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Radio Setup
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Pink noise
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Welcome, Morse code
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Police radio
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Horrifying throbbing low-frequency rumble
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Low frequency electronic signal
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Transfer, Morse code, reception
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SOS signal transmission, Morse code
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Morse code transmission
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White noise
★★★★★