MacBook Pro - MacBook Pro noises

This is a query of nothing more than idle curiosity, i wish i could see a suitable thread to add it too rather than start a new:

My Macbook Pro makes an almost cute little popping noise, very faintly, every few minutes from inside the LH side of the case, it's not a problem, from my rudimentary knowledge of hardware i just can't imagine what makes this noise, any of you techs know?