Your ears are not a closed system. They are affected by congestion in your throat and nose and by environmental air pressure such as when you're flying, diving or climbing a mountain. Pressure in the ear can cause pain, a full feeling and popping.
Behind the ear drum is the Eustachian tube. Air enters the tube and is normally balanced between the outside air pressure and the middle ear.
The popping noise you hear is air going into the Eustachian tube.
Your nose and ears can become congested when you are sick. Congestion in the Eustachian tube can block air from coming and leaving normally and result in uneven air pressure. The uneven pressure can cause pain, popping or both.
Drinking a lot of fluids will help thin the mucous that builds up in the ear and nose when you're congested. Decongestants and decongestant nasal sprays may help decrease the congestion and reduce inflammation. Gargling with salt water also helps clear the tubes.
Yawning, swallowing and chewing gum may help you to pop the ears more easily.
If you have popping often or even when not sick or flying, see a doctor to make sure you do not have damaged ear drums.