By thermal noise, they don't mean the temperature going up and down with CPU load. Thermal noise comes from atoms jiggling, where the average kinetic energy of

By thermal noise, they don't mean the temperature going up and down with CPU load. Thermal noise comes from atoms jiggling, where the average kinetic energy of an atom is proportional to absolute temperature. If you put a sensitive-enough amplifier across a resistor passing current, it will detect individual electrons emerging from the maze of charges they just went through. It is very, very random.