Question

sound is produced by vibrations but pendulum vibrations cannot be heard Why

Answer 1

Humans can hear the sounds with frequency 20Hz to 20kHz. The frequency of pendulum is much lower than 20Hz. Its frequency is 0.5Hz. So we can't hear it.

Answer 2

The human ear can hear the sounds which have the frequency between 20cycles/s to 20000cycles/sec. 
The frequency of pendulum vibration is 0.5cycles/sec which is less than our range thus we cannot hear it