These were DTMF tones for cueing. (was: Sound heard on basic cable........)
Those Digital Multi-Frequency Tones (DTMF) were/are generally for cueing the headends of cable systems to insert local spots (commercials). Usually, they were sent about 8 seconds before the local break. Originally, networks like CNN Headline News would insert those tones during a bumper promo just before the break on the main audio channel ( (-bleep- DTMF tones in rapid succession) "This is CNN Headline News.... you're in tune with the world....."). Later, most of these DTMF tones (same tones as a touch-tone phone, but faster) would be sent over the "cue" channel, of which only the headend receivers would "hear" them to key up a local break. Very few, if any, cable networks put the cue tones on the main channel today.