I’m routinely annoyed by what I am reasonably sure is the sound of my MacBook Pro’s heat pipes and other warm components expanding and contracting, making creaking noises shortly after starting it up on a winter day. Then there’s the gentle gaseous hissing I would swear is the heat pipes condensing and evaporating (based on workload and laptop temperature concurrent with the noise) if I could think of a practical reason for them to be audible.