Some combinations of CPUs and motherboards in current laptops are producing an annoying, "intermittent high frequency buzzing noise" that has actually been identified as a problem by Dell. The problem was first noted last year in MacBook Pros, but has since been heard in some Dell and HP notebooks as well. The afflicted models run Intel Core 2 processors. Dell's tech support notes state,
The noise has been isolated to the processors power circuit and is only audible when the processor is in C3 (clock-stopped) power state. In C3 power state, notebook power is conserved resulting in reduced overall chassis heat and extended battery life. The varying voltage changes to the components in the processor's power circuit are caused by a phenomenon referred to as the piezoelectric effect. When a specific varying voltage is applied to these components, they begin to resonate producing sounds that fall within the frequency range of human hearing (15 20 KHz).
We're not certain that piezoelectric effect is the correct diagnosis, but it's clearly a problem in the voltage regulator circuits on the motherboards. Their suggested solution is to change device settings to prevent the processor from entering the C3 low power state... which is not particularly friendly to mobile computing under battery power. Cloneman, an SPCR forum member who brought this issue to our attention, describes a few other solutions in this forum thread.