Pretty much sums up the question. When your sinuses clear up, like, for example, when you take a hefty snort of oxymetazoline (Dristan), what on earth are those crackly, popping noises? What is causing those? Sure feels good, but what the hell are those strange sounds?
The paranasal sinuses are fixed spaces with openings into the nasal cavity.
As fluid drains out or air makes its way in, you might hear and feel a popping much the same way that fluid burbles out of any fixed enclosed space and air burbles in to equalize the pressure–say a bottle with a small neck. Because the fluid is viscous you might get a fairly sudden release as the pressure differentials equalize.
You also might want to quit shoving Rice Krispies up your nose, too.