Alternation
A metronome’s rhythm exists because there’s a sound followed by a moment of silence. Without alternation between sound and silence there can’t be any rhythm. There are many types of alternation. There is alternation between sound and silence, high- and low-pitched sounds, or loud and quiet sounds, for example. The sounds of a ticking clock, walking feet, and a bouncing ball reveal their rhythms because of the alternation. White noise, like the sounds of a constantly humming fan or a waterfall, doesn’t have rhythm because it lacks alternation.