When I leave my cell phone close to the computer, the sound system occasionally makes a strange noise. The same noise occurs when I receive a call or when I make the call. What it is?"
The cables of the small speakers of computer sound systems are not well shielded and can pick up intense RF signals emitted nearby. These signals are those sent by your cell phone. These digital signals are emitted both when the cell phone transmits and is about to receive a call, because at this moment it responds to the station confirming the call and therefore the noise occurs. Also, from time to time the cell phone communicates with the station, informing where it is. This noise is nothing more than the digital signal that contains the message.
