Cronek reports discovering a security flaw in the microphone on certain HP laptops. Using his RTL-SDR dongle and SDR# he observed unusual signals around 24 MHz. He discovered that his HP EliteBook 8460p laptop computer was continuously and unintentionally transmitting the audio from the built in microphone at 24 MHz in FM modulation anytime the laptop was turned on.

http://dangerousprototypes.com/2013/09/04/hp-laptop-security-flaw-discovered-using-rtl-sdr/