Hit one of the pipes hanging on the stand. Listen to the sound it makes. Dampen its vibration with your hand. Do the same with another pipe. The two emit sounds with slightly different pitches. Quickly hit one pipe after another. You will hear the sound cyclically growing and decreasing in intensity – that is, rumbling.
How does it work
Soundwaves can merge. The new wave has properties that depend on the parameters of the component waves. When waves with similar but not identical frequencies are combined, we can hear rumblings whose intensity varies cyclically between a minimum and a maximal value.