Two pure sine waves at slightly different frequencies, played together, produce a beat — a slow throb at the difference frequency. Neither tone contains rhythm. The throb isn’t in either of them. It’s in the relation between them.
If one wave is at 440 Hz and the other at 442 Hz, you hear a 2 Hz pulse. At some moments the waves align and reinforce; at others they cancel. The combined amplitude swells and quiets twice a second.
As the difference grows the beat speeds up. Around fifteen pulses a second you stop hearing distinct throbs and start hearing roughness — the beating has gone too fast for the ear to track and arrives as a kind of harshness. Push the difference higher and you stop hearing one wobbling tone and start hearing two.
Piano tuners use the slow part of this. You play a tuning fork and a string together. If the string is sharp or flat by a small amount, you hear the beat. As you tighten or loosen the string, the beat slows. When it stops, you’re matched. The ear is bad at absolute pitch and exquisite at detecting beating.