This is the sound of a cork popping out of a bottle, spread out over two instruments and four octaves. To get the popping sound, the player first inserts the plungers into the tubes, then pulls them out quickly. The playing action is slow – this instrument is hard to play rapidly.
The video linked to the icon below shows suction pops played alone. You can also go to this video, which shows suction pops in ensemble with other instruments.