Fansmitter is a new acoustic data exfiltration method devised by a group of researchers from Ben-Gurion University of the Negev. The method leverages on a mobile phone placed in the vicinity of the targeted machine to monitor the computer fans. The malicious code developed by the researchers can use the devices fans to exfiltrate data, with this method it sends out bits of data to the mobile phone or any other computer equipped with a microphone. The noise produced by the fan is included in the 100-600 Hz range, which is audible by humans.”]
Source: https://securityaffairs.co/wordpress/48678/hacking/fansmitter-exfiltrating-data.html