At which point they'd find out how to manipulate specific capacitors on the board to emit noise. Did you ever hold a cheap backlit wristwatch up to your ear as a kid? That sound.

Perhaps a cheaper way to solve this is to hide the physical case inside a locked cabinet, with a ventilation fan? Most users of secure systems don't need physical access to the case, and the ventilator should drown out any signal noise from such malware.