hack an air-gap computer using the sounds made by a hard drive actuator.

A small team of researchers at Ben-Gurion University in Israel has found a way to hack an air-gap computer using the sounds made by a hard drive actuator. They describe the technique and possible ways it might be used in a paper they have uploaded to the preprint server arXiv.
One way to keep hackers from stealing data from your computer is to unplug it from the Internet and to disable its WiFi, Bluetooth and speakers—creating an air gap between it and all other computer devices—at least according to conventional thinking. But now the team in Israel has shown even that may not be enough because they have found a way that hackers can read information off your computer then broadcast it using the noises your computer hard drive makes as it reads and writes information.
The idea works like this; a hacker somehow manages to install a small bit of malware onto your supposedly secure computer—that code reads data from the computer, i.e. the hard drive, keystrokes as a user types in a password or information it finds in memory, etc., and then uses a special algorithm to convert that message into a sound signal by manipulating the actuator used by the to move the head to different parts of the drive below it. That sound signal can then be picked up by any smart electronic device, such as a phone, and decoded revealing the data sent from the computer. The researchers offer proof in the form of a video they have uploaded to YouTube.
The method does have its limitations, of course, the sound produced by an actuator is pretty soft, so much so that a reader would have to be no more than six feet away and it has an extremely slow transmission rate—approximately 180 bits per minute (it would take approximately 25 minutes to transmit a 4,096-bit encryption key, for example) which would make it impractical for larger files. Still the researchers suggest there are some applications where their technique would be useful—in the world of spies and government covert operations, it is assumed.