It could be excess wax your ear is trying to clear out. Try some ear wax removal. If you live in a higher altitude, then consider moving to a lower altitude. You can also put cotton balls or ear plugs to temporarily stop the popping.
Hold your nose closed and close Your mouth, too. Then proceed to force air into your nose as if you were trying to force your fingers off it to manually pop your ears. This could be a sign of Eustachian tube dysfunction.