Human ear can hear the lowest frequency about 20 Hz, this is 20 vibrations per second. Vibrations of pendulum are very small and frequency is less than 20 Hz, so we can't hear it.
It does, but the sound is at a frequency that's too low for you to hear. If you could build a pendulum short enough so that it made 20 or more complete swings every second, THEN you'd begin to hear sound from it.
