This is a peculiar phenomenon associated with cymbalta withdrawal. You know how the ambient hiss of background noise changes if you clench your teeth? Brain zaps are very similar sort of throbs, often several in rapid succession, and sound remarkably like the popping noises from the fire swamp in The Princess Bride. They're not painful, exactly, but they're stress-inducing and very unpleasant. You'll be sitting there just fine, then suddenly "PFF! PFFPFF!" and everything from your ears backward and halfway down your shoulders is tensed up.