Crypto Boffins - led by Adi Shamir of RSA fame - are investigating whether it might be possible to gain valuable clues about private encryption keys simply by listening to a targeted computer.
The sounds made by capacitors on motherboards might, in theory, give attackers code-breaking clues in much the same way electro-magnetic leakage or power fluctuations can be used in so-called "side-channel" attacks on secure systems. These kinds of attacks are well beyond the capability of your average hacker but they do have applications in the design of tamper-resistant systems. The research shows that cryptanalysts are prepared to think of any possible attack vector in their quest to ensure cryptographic systems remain secure.
Preliminary work by Adi Shamir and Eran Tromer of the Weizmann Institute in Israel showed that "acoustic emanations from personal computers are a surprisingly rich source of information on CPU activity". For instance the researchers have found that RSA signature/decryption sounds different for different secret keys and that monitoring audio signals can reveal the time of a decryption operation, useful in timing attacks, especially when an attacker can affect that input data of a cryptographic operation. The audio signals of interest are well above the frequency generated by fans and thus easily filtered out. A proof-of-concept presentation on the research can be found here. ®