The noise your computer makes isn't just sound. They can actually reveal the RSA keys it is using. That's what a team of researchers from Tel Aviv university found, after analyzing sound recordings made close to computers.

They also discovered that they can recover private RSA keys from that. They were able to tell if computers were adding, or multiplying, which further allowed them to determine if one of a number of RSA keys were being used by listening to the encryption of a fixed message using each key in turn.

Naked Security explains:
The authors ultimately went much further, contriving a way in which a particular email client, bombarded with thousands of carefully-crafted encrypted messages, might end up leaking its entire RSA private key, one bit at a time.
The technique only works with GnuPG 1.4.x RSA encryption software, for now, but thankfully that is out of date. [Tel Aviv university via Naked Security]